Fix memleak in zpl_parse_options

strsep() will advance tmp_mntopts, and will change it to NULL on last
iteration.  This will cause strfree(tmp_mntopts) to not free anything.

unreferenced object 0xffff8800883976c0 (size 64):
  comm "mount.zfs", pid 3361, jiffies 4294931877 (age 1482.408s)
  hex dump (first 32 bytes):
    72 77 00 73 74 72 69 63 74 61 74 69 6d 65 00 7a  rw.strictatime.z
    66 73 75 74 69 6c 00 6d 6e 74 70 6f 69 6e 74 3d  fsutil.mntpoint=
  backtrace:
    [<ffffffff81810c4e>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff811f9cac>] __kmalloc+0x16c/0x250
    [<ffffffffc065ce9b>] strdup+0x3b/0x60 [spl]
    [<ffffffffc080fad6>] zpl_parse_options+0x56/0x300 [zfs]
    [<ffffffffc080fe46>] zpl_mount+0x36/0x80 [zfs]
    [<ffffffff81222dc8>] mount_fs+0x38/0x160
    [<ffffffff81240097>] vfs_kern_mount+0x67/0x110
    [<ffffffff812428e0>] do_mount+0x250/0xe20
    [<ffffffff812437d5>] SyS_mount+0x95/0xe0
    [<ffffffff8181aff6>] entry_SYSCALL_64_fastpath+0x1e/0xa8
    [<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4706
Issue openzfs#4708

module/zfs/zpl_super.c

@@ -336,12 +336,12 @@ zpl_parse_options(char *osname, char *mntopts, zfs_mntopts_t *zmo,
 
 	if (mntopts) {
 		substring_t args[MAX_OPT_ARGS];
-		char *tmp_mntopts, *p;
+		char *tmp_mntopts, *p, *t;
 		int token;
 
-		tmp_mntopts = strdup(mntopts);
+		t = tmp_mntopts = strdup(mntopts);
 
-		while ((p = strsep(&tmp_mntopts, ",")) != NULL) {
+		while ((p = strsep(&t, ",")) != NULL) {
 			if (!*p)
 				continue;