Fix multiple issues in scrub_mirror:scrub_mirror_003_pos.

This test has never worked correctly.

tests/sys/cddl/zfs/tests/scrub_mirror/scrub_mirror_common.kshlib

• Fix usage of fill_fs. The arguments were passed in the wrong

	  order, resulting in writes with absurdly low block sizes, causing
	  the test to time out.

• Look for the files in the correct location. fill_fs creates its

	  files subdirectories, not directly in the directory passed as the
	  first argument.

• Don't bother to checksum nonexistent files
• Remove an extraneous umount command
• Remove some kshisms

tests/sys/cddl/zfs/tests/scrub_mirror/default.cfg
Define TESTFILE

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