Pack dmu_buf_impl_t by 16 bytes

On 64bit FreeBSD this reduces one from 296 to 280 bytes.  On small
block workloads dbufs may consume gigabytes of ARC, and this saves
5% of it.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16684

include/sys/dbuf.h

@@ -264,6 +264,27 @@ typedef struct dmu_buf_impl {
 	 */
 	uint8_t db_level;
 
+	/* This block was freed while a read or write was active. */
+	uint8_t db_freed_in_flight;
+
+	/*
+	 * Evict user data as soon as the dirty and reference counts are equal.
+	 */
+	uint8_t db_user_immediate_evict;
+
+	/*
+	 * dnode_evict_dbufs() or dnode_evict_bonus() tried to evict this dbuf,
+	 * but couldn't due to outstanding references.  Evict once the refcount
+	 * drops to 0.
+	 */
+	uint8_t db_pending_evict;
+
+	/* Number of TXGs in which this buffer is dirty. */
+	uint8_t db_dirtycnt;
+
+	/* The buffer was partially read.  More reads may follow. */
+	uint8_t db_partial_read;
+
 	/*
 	 * Protects db_buf's contents if they contain an indirect block or data
 	 * block of the meta-dnode. We use this lock to protect the structure of
@@ -288,6 +309,9 @@ typedef struct dmu_buf_impl {
 	 */
 	dbuf_states_t db_state;
 
+	/* In which dbuf cache this dbuf is, if any. */
+	dbuf_cached_state_t db_caching_status;
+
 	/*
 	 * Refcount accessed by dmu_buf_{hold,rele}.
 	 * If nonzero, the buffer can't be destroyed.
@@ -304,39 +328,10 @@ typedef struct dmu_buf_impl {
 	/* Link in dbuf_cache or dbuf_metadata_cache */
 	multilist_node_t db_cache_link;
 
-	/* Tells us which dbuf cache this dbuf is in, if any */
-	dbuf_cached_state_t db_caching_status;
-
 	uint64_t db_hash;
 
-	/* Data which is unique to data (leaf) blocks: */
-
 	/* User callback information. */
 	dmu_buf_user_t *db_user;
-
-	/*
-	 * Evict user data as soon as the dirty and reference
-	 * counts are equal.
-	 */
-	uint8_t db_user_immediate_evict;
-
-	/*
-	 * This block was freed while a read or write was
-	 * active.
-	 */
-	uint8_t db_freed_in_flight;
-
-	/*
-	 * dnode_evict_dbufs() or dnode_evict_bonus() tried to
-	 * evict this dbuf, but couldn't due to outstanding
-	 * references.  Evict once the refcount drops to 0.
-	 */
-	uint8_t db_pending_evict;
-
-	uint8_t db_dirtycnt;
-
-	/* The buffer was partially read.  More reads may follow. */
-	uint8_t db_partial_read;
 } dmu_buf_impl_t;
 
 #define	DBUF_HASH_MUTEX(h, idx) \