xfs: test xfs_copy doesn't do cached read before libxfs_mount

xfs: test xfs_copy doesn't do cached read before libxfs_mount

This is a regression test for an xfs_copy fix that ensures that it
doesn't perform a cached read of an XFS filesystem prior to initializing
libxfs, since the xfs_mount (and hence the buffer cache) isn't set up
yet.

[zlang: change the supported fs from generic to xfs]

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