_scratch_xfs_mdrestore: Pass scratch log device when applicable

_scratch_xfs_mdrestore: Pass scratch log device when applicable

Metadump v2 supports dumping contents of an external log device. This commit
modifies _scratch_xfs_mdrestore() and _xfs_mdrestore() to be able to restore
metadump files which contain data from external log devices.

The callers of _scratch_xfs_mdrestore() must set the value of $SCRATCH_LOGDEV
only when all of the following conditions are met:
1. Metadump is in v2 format.
2. Metadump has contents dumped from an external log device.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>