check: document mkfs.xfs reliance on fstests exports

check: document mkfs.xfs reliance on fstests exports

There are a number of fstests that employ special (and now unsupported)
XFS filesystem configurations to perform testing in a controlled
environment.  The presence of the QA_CHECK_FS and MSGVERB variables are
used by mkfs.xfs to detect that it's running inside fstests, which
enables the unsupported configurations.  Nobody else should be using
filesystems with tiny logs, non-redundant superblocks, or smaller than
the (new) minimum supported size.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>