ext4: fix potential deadlock in ext4_evict_inode()

ext4: fix potential deadlock in ext4_evict_inode()

Commit 2581fdc810 moved ext4_ioend_wait() from ext4_destroy_inode() to
ext4_evict_inode().  It also added code to explicitly call
ext4_flush_completed_IO(inode):

	mutex_lock(&inode->i_mutex);
	ext4_flush_completed_IO(inode);
	mutex_unlock(&inode->i_mutex);

Unfortunately, we can't take the i_mutex lock in ext4_evict_inode()
without potentially causing a deadlock.

Fix this by removing the code sequence altogether.  This may result in
ext4_evict_inode() taking longer to complete, but that's ok, we're not
in a rush here.  That just means we have to wait until the workqueue
is scheduled, which is OK; there's nothing that says we have to do
this work on the current thread, which would require taking a lock
that might lead to a deadlock condition.

See Kernel Bugzilla #41682 for one example of the circular locking
problem that arise.  Another one can be seen here:

=======================================================
[ INFO: possible circular locking dependency detected ]
3.1.0-rc3-00012-g2a22fc1 #1839
-------------------------------------------------------
dd/7677 is trying to acquire lock:
 (&type->s_umount_key#18){++++..}, at: [<c021ea77>] writeback_inodes_sb_if_idle+0x26/0x3d

but task is already holding lock:
 (&sb->s_type->i_mutex_key#3){+.+.+.}, at: [<c01d5956>] generic_file_aio_write+0x52/0xba

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&sb->s_type->i_mutex_key#3){+.+.+.}:
       [<c018eb02>] lock_acquire+0x99/0xbd
       [<c06a53b5>] __mutex_lock_common+0x33/0x2fb
       [<c06a572b>] mutex_lock_nested+0x26/0x2f
       [<c026c2db>] ext4_evict_inode+0x3e/0x2bd
       [<c0214bb0>] evict+0x8e/0x131
       [<c0214de6>] dispose_list+0x36/0x40
       [<c0215239>] evict_inodes+0xcd/0xd5
       [<c0204a23>] generic_shutdown_super+0x3d/0xaa
       [<c0204ab2>] kill_block_super+0x22/0x5e
       [<c0204cb8>] deactivate_locked_super+0x22/0x4e
       [<c02055b2>] deactivate_super+0x3d/0x43
       [<c0218427>] mntput_no_expire+0xda/0xdf
       [<c0219486>] sys_umount+0x286/0x2ab
       [<c02194bd>] sys_oldumount+0x12/0x14
       [<c06a6ac5>] syscall_call+0x7/0xb

-> #0 (&type->s_umount_key#18){++++..}:
       [<c018e262>] __lock_acquire+0x967/0xbd2
       [<c018eb02>] lock_acquire+0x99/0xbd
       [<c06a5991>] down_read+0x28/0x65
       [<c021ea77>] writeback_inodes_sb_if_idle+0x26/0x3d
       [<c0269630>] ext4_nonda_switch+0xd0/0xe1
       [<c026e953>] ext4_da_write_begin+0x3c/0x1cf
       [<c01d46ad>] generic_file_buffered_write+0xc0/0x1b4
       [<c01d58d3>] __generic_file_aio_write+0x254/0x285
       [<c01d596e>] generic_file_aio_write+0x6a/0xba
       [<c026732f>] ext4_file_write+0x1d6/0x227
       [<c0202789>] do_sync_write+0x8f/0xca
       [<c02030d5>] vfs_write+0x85/0xe3
       [<c02031d4>] sys_write+0x40/0x65
       [<c06a6ac5>] syscall_call+0x7/0xb

https://bugzilla.kernel.org/show_bug.cgi?id=41682

Cc: stable@kernel.org
Cc: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>