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

module/zfs/zfs_vnops.c

@@ -85,6 +85,7 @@ zfs_fsync(znode_t *zp, int syncflag, cred_t *cr)
 static int
 zfs_holey_common(znode_t *zp, ulong_t cmd, loff_t *off)
 {
+	zfs_locked_range_t *lr;
 	uint64_t noff = (uint64_t)*off; /* new offset */
 	uint64_t file_sz;
 	int error;
@@ -100,12 +101,18 @@ zfs_holey_common(znode_t *zp, ulong_t cmd, loff_t *off)
 	else
 		hole = B_FALSE;
 
+	/* Flush any mmap()'d data to disk */
+	if (zn_has_cached_data(zp))
+		zn_flush_cached_data(zp, B_FALSE);
+
+	lr = zfs_rangelock_enter(&zp->z_rangelock, 0, file_sz, RL_READER);
 	error = dmu_offset_next(ZTOZSB(zp)->z_os, zp->z_id, hole, &noff);
+	zfs_rangelock_exit(lr);
 
 	if (error == ESRCH)
 		return (SET_ERROR(ENXIO));
 
-	/* file was dirty, so fall back to using generic logic */
+	/* File was dirty, so fall back to using generic logic */
 	if (error == EBUSY) {
 		if (hole)
 			*off = file_sz;