xfs: fix memcpy fortify errors in RUI log format copying

Starting in 6.1, CONFIG_FORTIFY_SOURCE checks the length parameter of
memcpy.  Since we're already fixing problems with BUI item copying, we
should fix it everything else.

Refactor the xfs_rui_copy_format function to handle the copying of the
head and the flex array members separately.  While we're at it, fix a
minor validation deficiency in the recovery function.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>