Revert Consolidate arc_buf allocation checks

This reverts commit 13fac09868b4e4e08cc3ef7b937ac277c1c407b1.

Per the discussion in #11531, the reverted commit---which intended only
to be a cleanup commit---introduced a subtle, unintended change in
behavior.

Care was taken to partially revert and then reapply 10b3c7f5e4
which would otherwise have caused a conflict. These changes were
squashed in to this commit.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Suggested-by: @chrisrd
Suggested-by: robn@despairlabs.com
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #11531
Closes #12227