Remove vn_set_fs_pwd()/vn_set_pwd() (no need to be at / during insmod)

Per suggestion from @behlendorf in #8777, remove vn_set_fs_pwd() and
vn_set_pwd() which are only used in zfs_ioctl.c:_init() while loading
zfs.ko.

The rest of initialization functions being called here after cwd set
to / don't depend on cwd of the process except for spa_config_load().
spa_config_load() uses a relative path ".//etc/zfs/zpool.cache" when
`rootdir` is non-NULL, which is "/etc/zfs/zpool.cache" given cwd is /,
so just unconditionally use the absolute path without "./", so that
`vn_set_pwd("/")` as well as the entire functions can be removed.
This is also what FreeBSD does.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8826

module/zfs/zfs_ioctl.c

@@ -7380,13 +7380,6 @@ _init(void)
 {
 	int error;
 
-	error = -vn_set_pwd("/");
-	if (error) {
-		printk(KERN_NOTICE
-		    "ZFS: Warning unable to set pwd to '/': %d\n", error);
-		return (error);
-	}
-
 	if ((error = -zvol_init()) != 0)
 		return (error);