bcachefs: avoid out-of-bounds in split_devs

Calling mount with an empty source string causes an out-of-bounds error
in split_devs. Check the length of the source string to avoid this.
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -32,6 +32,7 @@
 #include <linux/random.h>
 #include <linux/seq_file.h>
 #include <linux/statfs.h>
+#include <linux/string.h>
 #include <linux/xattr.h>
 
 static struct kmem_cache *bch2_inode_cache;
@@ -1324,6 +1325,9 @@ static char **split_devs(const char *_dev_name, unsigned *nr)
 	char *dev_name = NULL, **devs = NULL, *s;
 	size_t i, nr_devs = 0;
 
+	if (strlen(_dev_name) == 0)
+		return NULL;
+
 	dev_name = kstrdup(_dev_name, GFP_KERNEL);
 	if (!dev_name)
 		return NULL;