Skip dbuf_evict_one() from dbuf_evict_notify() for reclaim thread

Avoid calling dbuf_evict_one() from memory reclaim contexts (e.g. Linux
kswapd, FreeBSD pagedaemon). This prevents deadlock caused by reclaim
threads waiting for the dbuf hash lock in the call sequence:
dbuf_evict_one -> dbuf_destroy -> arc_buf_destroy

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Kaitlin Hoang <kthoang@amazon.com>
Closes #17561

include/sys/zfs_context.h

@@ -224,6 +224,11 @@ typedef pthread_t	kthread_t;
 #define	thread_join(t)	pthread_join((pthread_t)(t), NULL)
 
 #define	newproc(f, a, cid, pri, ctp, pid)	(ENOSYS)
+/*
+ * Check if the current thread is a memory reclaim thread.
+ * Always returns false in userspace (no memory reclaim thread).
+ */
+#define	current_is_reclaim_thread()	(0)
 
 /* in libzpool, p0 exists only to have its address taken */
 typedef struct proc {