device_get_path(): take sbuf directly

This allows to fix a bug where sbuf allocation done in the context of
dev_wired_cache_match() must use non-sleepable allocations.

Suggested by: jhb
Reviewed by: jhb, takawata
Discussed with: imp
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36899