xfs: btree format inode forks can have zero extents

xfs/538 is assert failing with this trace when testing with
directory block sizes of 64kB:

XFS: Assertion failed: !xfs_need_iread_extents(ifp), file: fs/xfs/libxfs/xfs_bmap.c, line: 608
....
Call Trace:
 xfs_bmap_btree_to_extents+0x2a9/0x470
 ? kmem_cache_alloc+0xe7/0x220
 __xfs_bunmapi+0x4ca/0xdf0
 xfs_bunmapi+0x1a/0x30
 xfs_dir2_shrink_inode+0x71/0x210
 xfs_dir2_block_to_sf+0x2ae/0x410
 xfs_dir2_block_removename+0x21a/0x280
 xfs_dir_removename+0x195/0x1d0
 xfs_remove+0x244/0x460
 xfs_vn_unlink+0x53/0xa0
 ? selinux_inode_unlink+0x13/0x20
 vfs_unlink+0x117/0x220
 do_unlinkat+0x1a2/0x2d0
 __x64_sys_unlink+0x42/0x60
 do_syscall_64+0x3a/0x70
 entry_SYSCALL_64_after_hwframe+0x44/0xae

This is a check to ensure that the extents have been read into
memory before we are doing a ifork btree manipulation. This assert
is bogus in the above case.

We have a fragmented directory block that has more extents in it
than can fit in extent format, so the inode data fork is in btree
format. xfs_dir2_shrink_inode() asks to remove all remaining 16
filesystem blocks from the inode so it can convert to short form,
and __xfs_bunmapi() removes all the extents. We now have a data fork
in btree format but have zero extents in the fork. This incorrectly
trips the xfs_need_iread_extents() assert because it assumes that an
empty extent btree means the extent tree has not been read into
memory yet. This is clearly not the case with xfs_bunmapi(), as it
has an explicit call to xfs_iread_extents() in it to pull the
extents into memory before it starts unmapping.

Also, the assert directly after this bogus one is:

	ASSERT(ifp->if_format == XFS_DINODE_FMT_BTREE);

Which covers the context in which it is legal to call
xfs_bmap_btree_to_extents just fine. Hence we should just remove the
bogus assert as it is clearly wrong and causes a regression.

The returns the test behaviour to the pre-existing assert failure in
xfs_dir2_shrink_inode() that indicates xfs_bunmapi() has failed to
remove all the extents in the range it was asked to unmap.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/libxfs/xfs_bmap.c

@@ -605,7 +605,6 @@ xfs_bmap_btree_to_extents(
 
 	ASSERT(cur);
 	ASSERT(whichfork != XFS_COW_FORK);
-	ASSERT(!xfs_need_iread_extents(ifp));
 	ASSERT(ifp->if_format == XFS_DINODE_FMT_BTREE);
 	ASSERT(be16_to_cpu(rblock->bb_level) == 1);
 	ASSERT(be16_to_cpu(rblock->bb_numrecs) == 1);