Fix build on FreeBSD

Constify some variables after d1807f1.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by:  Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #14656

module/os/freebsd/zfs/spa_os.c

@@ -184,7 +184,7 @@ spa_import_rootpool(const char *name, bool checkpointrewind)
 	spa_t *spa;
 	vdev_t *rvd;
 	nvlist_t *config, *nvtop;
-	char *pname;
+	const char *pname;
 	int error;
 
 	/*

module/os/freebsd/zfs/vdev_geom.c

@@ -543,7 +543,7 @@ process_vdev_config(nvlist_t ***configs, uint64_t *count, nvlist_t *cfg,
 	uint64_t pool_guid;
 	uint64_t vdev_guid;
 	uint64_t id, txg, known_txg;
-	char *pname;
+	const char *pname;
 
 	if (nvlist_lookup_string(cfg, ZPOOL_CONFIG_POOL_NAME, &pname) != 0 ||
 	    strcmp(pname, name) != 0)

module/os/freebsd/zfs/zfs_ioctl_os.c

@@ -99,7 +99,7 @@ zfs_ioc_nextboot(const char *unused, nvlist_t *innvl, nvlist_t *outnvl)
 	char name[MAXNAMELEN];
 	spa_t *spa;
 	vdev_t *vd;
-	char *command;
+	const char *command;
 	uint64_t pool_guid;
 	uint64_t vdev_guid;
 	int error;