ARC: Drop different size headers for crypto

To reduce memory usage ZFS crypto allocated bigger by 56 bytes ARC
headers only when specific block was encrypted on disk. It was a
nice optimization, except in some cases the code reallocated them
on fly, that invalidated header pointers from the buffers. Since
the buffers use different locking, it created number of races, that
were originally covered (at least partially) by b_evict_lock, used
also to protection evictions. But it has gone as part of #14340.
As result, as was found in #15293, arc_hdr_realloc_crypt() ended
up unprotected and causing use-after-free.

Instead of introducing some even more elaborate locking, this patch
just drops the difference between normal and protected headers. It
cost us additional 56 bytes per header, but with couple patches
saving 24 bytes, the net growth is only 32 bytes with total header
size of 232 bytes on FreeBSD, that IMHO is acceptable price for
simplicity. Additional locking would also end up consuming space,
time or both.

Reviewe-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15293
Closes #15347