zstd: don't call zstd_mempool_reap if there are no buffers (#16302)

zfs_zstd_cache_reap_now is issued every second.

zstd_mempool_reap checks for both pool existence and buffer count, but
that's still 2 func calls which are trivially avoidable.

With clang it even avoids pushing the stack pointer (but still suffers
the mispredict due to a forward jump, not modified in case someone is
using zstd):

<+0>:     cmpq   $0x0,0x0(%rip)        # <zfs_zstd_cache_reap_now+8>
<+8>:     je     0x217de4 <zfs_zstd_cache_reap_now+36>
<+10>:    push   %rbp
<+11>:    mov    %rsp,%rbp
<+14>:    mov    0x0(%rip),%rdi        # <zfs_zstd_cache_reap_now+21>
<+21>:    call   0x217df0 <zstd_mempool_reap>
<+26>:    mov    0x0(%rip),%rdi        # <zfs_zstd_cache_reap_now+33>
<+33>:    pop    %rbp
<+34>:    jmp    0x217df0 <zstd_mempool_reap>
<+36>:    ret

Preferably the call would not be made to begin with if zstd is not used,
but this retains all the logic confined to zstd code.

Sponsored by:	Rubicon Communications, LLC ("Netgate")
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

module/zstd/zfs_zstd.c

@@ -844,6 +844,13 @@ zstd_mempool_deinit(void)
 void
 zfs_zstd_cache_reap_now(void)
 {
+
+	/*
+	 * Short-circuit if there are no buffers to begin with.
+	 */
+	if (ZSTDSTAT(zstd_stat_buffers) == 0)
+		return;
+
 	/*
 	 * calling alloc with zero size seeks
 	 * and releases old unused objects