Commit 01239d77 ("xfs: fix a null pointer dereference in
xfs_bmap_extents_to_btree") attempted to fix a null pointer
dreference when a fuzzing corruption of some kind was found.
This fix was flawed, resulting in assert failures like:

XFS: Assertion failed: ifp->if_broot == NULL, file: fs/xfs/libxfs/xfs_bmap.c, line: 715
.....
Call Trace:
  xfs_bmap_extents_to_btree+0x6b9/0x7b0
  __xfs_bunmapi+0xae7/0xf00
  ? xfs_log_reserve+0x1c8/0x290
  xfs_reflink_remap_extent+0x20b/0x620
  xfs_reflink_remap_blocks+0x7e/0x290
  xfs_reflink_remap_range+0x311/0x530
  vfs_dedupe_file_range_one+0xd7/0xe0
  vfs_dedupe_file_range+0x15b/0x1a0
  do_vfs_ioctl+0x267/0x6c0

The problem is that the error handling code now asserts that the
inode fork is not in btree format before the error handling code
undoes the modifications that put the fork back in extent format.
Fix this by moving the assert back to after the xfs_iroot_realloc()
call that returns the fork to extent format, and clean up the jump
labels to be meaningful.

Also, returning ENOSPC when xfs_btree_get_bufl() fails to
instantiate the buffer that was allocated (the actual fix in the
commit mentioned above) is incorrect. This is a fatal error - only
an invalid block address or a filesystem shutdown can result in
failing to get a buffer here.

Hence change this to EFSCORRUPTED so that the higher layer knows
this was a corruption related failure and should not treat it as an
ENOSPC error.  This should result in a shutdown (via cancelling a
dirty transaction) which is necessary as we do not attempt to clean
up the (invalid) block that we have already allocated.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>