osd: Constify signature of osd_register()

This makes it clear that 'methods' (if not NULL) points to an array that
won't be modified. Internally, this array is indeed copied into the
given OSD type's larger array of all methods for all slots.

Reviewed by: jamie
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46886