getblk: avoid bufobj interlock when waiting on buf lock

[rlibby]

Summary

Convert buf's b_lock to a lock_sleepgen so that in getblk we can avoid
the bufobj interlock when we have to sleep to acquire a buf lock.

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Again, work-in-progress. Looking for objections or early feedback.

[markj]

sys/sys/buf.h

321

[mckusick]

Sorry to be slow to comment. Like D45624, this change looks functionally correct. Again, the main issue for me is to understand what sort of workload it helps. Do you have an example benchmark whose performance is improved with this change?