getblk: fail faster with GB_LOCK_NOWAIT

If we asked not to wait on a lock, and then we failed to get a buf lock
because we would have had to wait, then just return the error. This
avoids taking the bufobj lock and a second trip to lockmgr.

Reviewed by: mckusick, kib, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45245