raid1: ensure write behind bio has less than BIO_MAX_VECS sectors

Merge Request Required Information

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2117034

Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=47196413

Upstream Status: Linus's git tree (linux-next)

Tested by reporter and fixed the issue.

Summary of Changes

We can't split write behind bio with more than BIO_MAX_VECS sectors, otherwise the below call trace was triggered because we could allocate oversized write behind bio later.

Approved Bugzilla Ticket

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Signed-off-by: Nigel Croxon ncroxon@redhat.com