fuse: Grab the fc->lock for inode updates in shortcircuit

The function fsstack_copy_inode_size uses the inode->i_lock to
serialize the inode updates. This lock is not used in the FUSE
filesystem and thus it is not enough to just grab this lock
before updating a FUSE inode.

Grab the fc->lock for inode updates in shortcircuit to ensure
that there are no races between inode size updates in 32 bit
architectures with SMP.

Change-Id: I83cb2380b6ca56768c06e70ef1bf9ea3976b514a
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>