Improve caching for dbuf prefetches

To avoid read errors with transaction open dmu_tx_check_ioerr()
is used to read everything required in advance.  But there seems
to be a chance for the buffer to evicted from dbuf cache in
between, which result in immediate eviction from ARC, which may
require additional disk read later in a place where error handling
is problematic.

To partially workaround this introduce a new flag DMU_IS_PREFETCH,
relayed to ARC as ARC_FLAG_PREFETCH | ARC_FLAG_PRESCIENT_PREFETCH,
making ARC delay eviction by at least several seconds, or till the
actual read inside the transaction, that will promote it to demand
access.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18160

include/sys/dmu.h

@@ -573,6 +573,7 @@ typedef enum dmu_flags {
 	DMU_PARTIAL_FIRST	= 1 << 7, /* First partial access. */
 	DMU_PARTIAL_MORE	= 1 << 8, /* Following partial access. */
 	DMU_KEEP_CACHING	= 1 << 9, /* Don't affect caching. */
+	DMU_IS_PREFETCH		= 1 << 10, /* This read is a prefetch. */
 } dmu_flags_t;
 
 /*