check: capture dmesg of mount failures if test fails

check: capture dmesg of mount failures if test fails

Capture the kernel output after a mount failure occurs.  If the test
itself fails, then keep the logging output for further diagnosis.  The
xunit.xsd update adds a <mount-failure> element to the xml output, whose
output is the mountfail file.

Note that because the .mountfail file is preserved by ./check, the new
selftest requires the user to check for the .mountfail file.  This is a
little awkward.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>