Fix getblk() with GB_NOCREAT returning false-negatives.

Fix getblk() with GB_NOCREAT returning false-negatives.

It is possible for a buf to be reassigned between the dirty and clean
lists while gbincore_unlocked() looks in each list.  Avoid creating
a buffer in that case and fallback to a locked lookup.

This fixes a regression from r363482.

More discussion on potential improvements to the clean and dirty lists
handling is in the review.

Reviewed by:	cem, kib, markj, vangyzen, rlibby
Reported by:	Suraj.Raju at dell.com
Submitted by:	Suraj.Raju at dell.com, cem, [based on both]
MFC after:	2 weeks
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D28375