generic/173: don't dump core when mwrite fails

generic/173: don't dump core when mwrite fails

In generic/173, we try to force a CoW to a mmap'd region to fail if
there's no space to actually stage the CoW operation.  That failure
comes in the form of a SIGBUS to xfs_io.  If the tester just happens
to have a nonzero coresize ulimit set, a core dump is generated and
the test is marked as having failed, even though the dump generation
is exactly the correct behavior.

Therefore, set the coresize ulimit to zero while calling
_mwrite_byte.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>