bcachefs fixes for 6.15-rc2

Mostly minor fixes.

Eric Biggers' crypto API conversion is included because of long standing
sporadic crashes - mostly, but not entirely syzbot - in the crypto API
code when calling poly1305, which have been nigh impossible to reproduce
and debug. His rework deletes the code where we've seen the crashes, so
either it'll be a fix or we'll end up with backtraces we can debug.
(Thanks Eric!).
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