fuse: fix hang on SMP

fuse: fix hang on SMP

Fuse didn't always call i_size_write() with i_mutex held which caused
rare hangs on SMP/32bit.  This bug has been present since fuse-2.2,
well before being merged into mainline.

The simplest solution is to protect i_size_write() with the
per-connection spinlock.  Using i_mutex for this purpose would require
some restructuring of the code and I'm not even sure it's always safe
to acquire i_mutex in all places i_size needs to be set.

Since most of vmtruncate is already duplicated for other reasons,
duplicate the remaining part as well, making all i_size_write() calls
internal to fuse.

Using i_size_write() was unnecessary in fuse_init_inode(), since this
function is only called on a newly created locked inode.

Reported by a few people over the years, but special thanks to Dana
Henriksen who was persistent enough in helping me debug it.

Adrian Bunk:
Backported to 2.6.16.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Adrian Bunk <bunk@stusta.de>