Remove lint ifdef checks in zdb and dbuf

This is effectively dead code for the Linux implementation which can
be removed to improve readability.  We want to linter to check the
real production/debug build as much as possible.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5722

module/zfs/dbuf.c

@@ -78,12 +78,10 @@ uint_t zfs_dbuf_evict_key;
 static boolean_t dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
 static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx);
 
-#ifndef __lint
 extern inline void dmu_buf_init_user(dmu_buf_user_t *dbu,
     dmu_buf_evict_func_t *evict_func_sync,
     dmu_buf_evict_func_t *evict_func_async,
     dmu_buf_t **clear_on_evict_dbufp);
-#endif /* ! __lint */
 
 /*
  * Global data structures and functions for the dbuf cache.