xfs: relax shortform directory size checks

Each of the four functions that operate on shortform directories checks
that the directory's di_size is at least as large as the shortform
directory header.  This is now checked by the inode fork verifiers
(di_size is used to allocate if_bytes, and if_bytes is checked against
the header structure size) so we can turn these checks into ASSERTions.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>