fs: Make write(2) interruptible by SIGKILL

fs: Make write(2) interruptible by SIGKILL

Currently write(2) to a file is not interruptible by a signal. Sometimes this
is desirable (e.g. when you want to quickly kill a process hogging your disk or
when some process gets blocked in balance_dirty_pages() indefinitely due to a
filesystem being in an error condition).

Reported-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Tested-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>