ARC: Remove b_cv from struct l1arc_buf_hdr

Earlier as part of #14123 I've removed one use of b_cv. This patch
reuses the same approach to remove the other one from much more
rare code path.

This saves 16 bytes of L1 ARC header on FreeBSD (reducing it from
200 to 184 bytes) and seems even more on Linux.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15340