mdrestore: Extract target device size verification into a function

mdrestore: Extract target device size verification into a function

A future commit will need to perform the device size verification on an
external log device. In preparation for this, this commit extracts the
relevant portions into a new function. No functional changes have been
introduced.

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>