xfs: fix xfs_bunmapi to allow unmapping of partial rt extents

When XFS_BMAPI_REMAP is passed to bunmapi, that means that we want to
remove part of a block mapping without touching the allocator.  For
realtime files with rtextsize > 1, that also means that we should skip
all the code that changes a partial remove request into an unwritten
extent conversion.  IOWs, bunmapi in this mode should handle removing
the mapping from the rt file and nothing else.

Note that XFS_BMAPI_REMAP callers are required to decrement the
reference count and/or free the space manually.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_bmap.c

@@ -5459,7 +5459,7 @@ __xfs_bunmapi(
 		if (del.br_startoff + del.br_blockcount > end + 1)
 			del.br_blockcount = end + 1 - del.br_startoff;
 
-		if (!isrt)
+		if (!isrt || (flags & XFS_BMAPI_REMAP))
 			goto delete;
 
 		mod = xfs_rtb_to_rtxoff(mp,
@@ -5477,7 +5477,7 @@ __xfs_bunmapi(
 				 * This piece is unwritten, or we're not
 				 * using unwritten extents.  Skip over it.
 				 */
-				ASSERT(end >= mod);
+				ASSERT((flags & XFS_BMAPI_REMAP) || end >= mod);
 				end -= mod > del.br_blockcount ?
 					del.br_blockcount : mod;
 				if (end < got.br_startoff &&