mm: Remove false WARN_ON from pagecache_isize_extended()

The WARN_ON checking whether i_mutex is held in
pagecache_isize_extended() was wrong because some filesystems (e.g.
XFS) use different locks for serialization of truncates / writes. So
just remove the check.
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

mm/truncate.c

@@ -755,7 +755,6 @@ void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to)
 	struct page *page;
 	pgoff_t index;
 
-	WARN_ON(!mutex_is_locked(&inode->i_mutex));
 	WARN_ON(to > inode->i_size);
 
 	if (from >= to || bsize == PAGE_CACHE_SIZE)