common: capture metadump output if xfs filesystem check fails

Capture metadump output when various userspace repair and checker tools
fail or indicate corruption, to aid in debugging.  We don't bother to
annotate xfs_check because it's bitrotting.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

README

@@ -109,6 +109,8 @@ Preparing system for tests:
              - Set TEST_FS_MODULE_RELOAD=1 to unload the module and reload
                it between test invocations.  This assumes that the name of
                the module is the same as FSTYP.
+             - Set DUMP_CORRUPT_FS=1 to record metadata dumps of XFS
+               filesystems if a filesystem check fails.
 
         - or add a case to the switch in common/config assigning
           these variables based on the hostname of your test