fuse: lock inode unconditionally in fuse_fallocate()

file_modified() must be called with inode lock held.  fuse_fallocate()
didn't lock the inode in case of just FALLOC_KEEP_SIZE flags value, which
resulted in a kernel Warning in notify_change().

Lock the inode unconditionally, like all other fallocate implementations
do.

Reported-by: Pengfei Xu <pengfei.xu@intel.com>
Reported-and-tested-by:  <syzbot+462da39f0667b357c4b6@syzkaller.appspotmail.com>
Fixes: 4a6f278d ("fuse: add file_modified() to fallocate")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>