icp: properly fix all RETs in x86_64 Asm code

Commit 43569ee37420 ("Fix objtool: missing int3 after ret warning")
addressed replacing all rets in x86 asm code to a macro in the
Linux kernel in order to enable SLS. That was done by copying the
upstream macro definitions and fixed objtool complaints.
Since then, several more mitigations were introduced, including
Rethunk. It requires to have a jump to one of the thunks in order
to work, so the RET macro was changed again. And, as ZFS code
didn't use the mainline defition, but copied it, this is currently
missing.

Objtool reminds about it time to time (Clang 16, CONFIG_RETHUNK=y):

fs/zfs/lua/zlua.o: warning: objtool: setjmp+0x25: 'naked' return
found in RETHUNK build
fs/zfs/lua/zlua.o: warning: objtool: longjmp+0x27: 'naked' return
found in RETHUNK build

Do it the following way:

• if we're building under Linux, unconditionally include <linux/linkage.h> in the related files. It is available in x86 sources since even pre-2.6 times, so doesn't need any conftests;
• then, if RET macro is available, it will be used directly, so that we will always have the version actual to the kernel we build;
• if there's no such macro, we define it as a simple ret, as it was on pre-SLS times.

This ensures we always have the up-to-date definition with no need
to update it manually, and at the same time is safe for the whole
variety of kernels ZFS module supports.
Then, there's a couple more "naked" rets left in the code, they're
just defined as:

.byte 0xf3,0xc3

In fact, this is just:

rep ret

rep ret instead of just ret seems to mitigate performance issues
on some old AMD processors and most likely makes no sense as of
today.
Anyways, address those rets, so that they will be protected with
Rethunk and SLS. Include <sys/asm_linkage.h> here which now always
has RET definition and replace those constructs with just RET.
This wipes the last couple of places with unpatched rets objtool's
been complaining about.

Reviewed-by: Attila Fülöp <attila@fueloep.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Closes #14035