Fix lseek(SEEK_DATA/SEEK_HOLE) mmap consistency

When using lseek(2) to report data/holes memory mapped regions of
the file were ignored.  This could result in incorrect results.
To handle this zfs_holey_common() was updated to asynchronously
writeback any dirty mmap(2) regions prior to reporting holes.

Additionally, while not strictly required, the dn_struct_rwlock is
now held over the dirty check to prevent the dnode structure from
changing.  This ensures that a clean dnode can't be dirtied before
the data/hole is located.  The range lock is now also taken to
ensure the call cannot race with zfs_write().

Furthermore, the code was refactored to provide a dnode_is_dirty()
helper function which checks the dnode for any dirty records to
determine its dirtiness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #11900
Closes #12724

tests/zfs-tests/include/commands.cfg

@@ -208,6 +208,7 @@ export ZFSTEST_FILES='badsend
     mktree
     mmap_exec
     mmap_libaio
+    mmap_seek
     mmapwrite
     nvlist_to_lua
     randfree_file

tests/zfs-tests/include/tunables.cfg

@@ -33,6 +33,7 @@ DEADMAN_FAILMODE		deadman.failmode		zfs_deadman_failmode
 DEADMAN_SYNCTIME_MS		deadman.synctime_ms		zfs_deadman_synctime_ms
 DEADMAN_ZIOTIME_MS		deadman.ziotime_ms		zfs_deadman_ziotime_ms
 DISABLE_IVSET_GUID_CHECK	disable_ivset_guid_check	zfs_disable_ivset_guid_check
+DMU_OFFSET_NEXT_SYNC		dmu_offset_next_sync		zfs_dmu_offset_next_sync
 INITIALIZE_CHUNK_SIZE		initialize_chunk_size		zfs_initialize_chunk_size
 INITIALIZE_VALUE		initialize_value		zfs_initialize_value
 KEEP_LOG_SPACEMAPS_AT_EXPORT	keep_log_spacemaps_at_export	zfs_keep_log_spacemaps_at_export