Throw const on some strings

In C, const indicates to the reader that mutation will not occur.
It can also serve as a hint about ownership.

Add const in a few places where it makes sense.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10997

module/zfs/dmu_objset.c

@@ -2413,7 +2413,7 @@ dmu_objset_is_snapshot(objset_t *os)
 }
 
 int
-dmu_snapshot_realname(objset_t *os, char *name, char *real, int maxlen,
+dmu_snapshot_realname(objset_t *os, const char *name, char *real, int maxlen,
     boolean_t *conflict)
 {
 	dsl_dataset_t *ds = os->os_dsl_dataset;