bcachefs: skip mount option handle for empty string.

The options parse in get_tree will split the options buffer, it will
get the empty string for last one by strsep(). After commit
ea0eeb8 ("bcachefs: reject unknown mount options") is merged,
unknown mount options is not allowed (here is empty string), and this
causes this errors. This can be reproduced just by the following steps:

    bcachefs format /dev/loop
    mount -t bcachefs -o metadata_target=loop1 /dev/loop1 /mnt/bcachefs/

Fixes: ea0eeb8 ("bcachefs: reject unknown mount options")
Signed-off-by: Hongbo Li <[email protected]>
Signed-off-by: Kent Overstreet <[email protected]>

Author: Hongbo Li

fs/bcachefs/opts.c

@@ -596,6 +596,9 @@ int bch2_parse_mount_opts(struct bch_fs *c, struct bch_opts *opts,
 	copied_opts_start = copied_opts;
 
 	while ((opt = strsep(&copied_opts, ",")) != NULL) {
+		if (!*opt)
+			continue;
+
 		name	= strsep(&opt, "=");
 		val	= opt;