ext4: fix a potential fiemap/page fault deadlock w/ inline_data

The ext4_inline_data_fiemap() function calls fiemap_fill_next_extent()
while still holding the xattr semaphore.  This is not necessary and it
triggers a circular lockdep warning.  This is because
fiemap_fill_next_extent() could trigger a page fault when it writes
into page which triggers a page fault.  If that page is mmaped from
the inline file in question, this could very well result in a
deadlock.

This problem can be reproduced using generic/519 with a file system
configuration which has the inline_data feature enabled.
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/inline.c

@@ -1890,12 +1890,12 @@ int ext4_inline_data_fiemap(struct inode *inode,
 	physical += (char *)ext4_raw_inode(&iloc) - iloc.bh->b_data;
 	physical += offsetof(struct ext4_inode, i_block);
 
-	if (physical)
-		error = fiemap_fill_next_extent(fieinfo, start, physical,
-						inline_len, flags);
 	brelse(iloc.bh);
 out:
 	up_read(&EXT4_I(inode)->xattr_sem);
+	if (physical)
+		error = fiemap_fill_next_extent(fieinfo, start, physical,
+						inline_len, flags);
 	return (error < 0 ? error : 0);
 }