Unify arc_prune_async() code

There is no sense to have separate implementations for FreeBSD and
Linux.  Make Linux code shared as more functional and just register
FreeBSD-specific prune callback with arc_add_prune_callback() API.

Aside of code cleanup this should fix excessive pruning on FreeBSD:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274698

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Mark Johnston <markj@FreeBSD.org>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15456

include/os/linux/zfs/sys/zpl.h

@@ -60,7 +60,7 @@ extern const struct file_operations zpl_file_operations;
 extern const struct file_operations zpl_dir_file_operations;
 
 /* zpl_super.c */
-extern void zpl_prune_sb(int64_t nr_to_scan, void *arg);
+extern void zpl_prune_sb(uint64_t nr_to_scan, void *arg);
 
 extern const struct super_operations zpl_super_operations;
 extern const struct export_operations zpl_export_operations;

include/sys/arc.h

@@ -81,7 +81,7 @@ typedef struct arc_prune arc_prune_t;
 typedef void arc_read_done_func_t(zio_t *zio, const zbookmark_phys_t *zb,
     const blkptr_t *bp, arc_buf_t *buf, void *priv);
 typedef void arc_write_done_func_t(zio_t *zio, arc_buf_t *buf, void *priv);
-typedef void arc_prune_func_t(int64_t bytes, void *priv);
+typedef void arc_prune_func_t(uint64_t bytes, void *priv);
 
 /* Shared module parameters */
 extern uint_t zfs_arc_average_blocksize;

include/sys/arc_impl.h

@@ -1065,7 +1065,6 @@ extern void arc_wait_for_eviction(uint64_t, boolean_t);
 
 extern void arc_lowmem_init(void);
 extern void arc_lowmem_fini(void);
-extern void arc_prune_async(uint64_t);
 extern int arc_memory_throttle(spa_t *spa, uint64_t reserve, uint64_t txg);
 extern uint64_t arc_free_memory(void);
 extern int64_t arc_available_memory(void);