common/rc: _add_dmesg_filter returns when RESULT_DIR is null

common/rc: _add_dmesg_filter returns when RESULT_DIR is null

I always hit below error on a system with readonly rootfs:

  ++ _xfs_prepare_for_eio_shutdown /dev/loop0
  ...
  ++ _add_dmesg_filter 'Internal error'
  ++ local 'regexp=Internal error'
  ++ local filter_file=/dmesg_filter
  ++ '[' '!' -e /dmesg_filter ']'
  ++ echo 'Internal error'
  ./common/rc: line 4716: /dmesg_filter: Read-only file system

The RESULT_DIR is null, due to xfstests/check calls _test_mount and
_scratch_mount before RESULT_DIR creation. And _test_mount does
_prepare_for_eio_shutdown -> _xfs_prepare_for_eio_shutdown ->
_add_dmesg_filter "Internal error" when RESULT_DIR is null.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>