bcachefs: fix memleak in bch2_split_devs

The pointer dev_name can be modified by strseq(),
then causes the memleak:

unreferenced object 0xffff9d08a2916c80 (size 32):
comm "mount.bcachefs", pid 9090, jiffies 4295856224 (age 17.564s)
hex dump (first 32 bytes):
2f 64 65 76 2f 6d 61 70 70 65 72 2f 74 65 73 74 /dev/mapper/test
2d 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 -0..............
backtrace:
[<00000000c5d3be7d>] __kmem_cache_alloc_node+0x1f3/0x2c0
[<0000000052215d26>] __kmalloc_node_track_caller+0x51/0x150
[<0000000069fea956>] kstrdup+0x32/0x60
[<000000000877fcf1>] bch2_split_devs+0x3f/0x150 [bcachefs]
[<000000007ee93204>] bch2_mount+0xcb/0x640 [bcachefs]
[<000000002dd1e04b>] legacy_get_tree+0x30/0x60
[<000000006afc31d3>] vfs_get_tree+0x28/0xf0
[<000000007b0c538e>] path_mount+0x475/0xb60
[<0000000092de5882>] __x64_sys_mount+0x105/0x140
[<0000000054fc05d8>] do_syscall_64+0x42/0xf0
[<00000000df584910>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

Fix it by copy pointer dev_name at beginning and free the copied
pointer at end.

Signed-off-by: Su Yue <[e-mail]>
Reviewed-by: Brian Foster <[e-mail]>
Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/util.c

···
 {
 	darray_init(ret);
 
-	char *dev_name = kstrdup(_dev_name, GFP_KERNEL), *s = dev_name;
+	char *dev_name, *s, *orig;
+
+	dev_name = orig = kstrdup(_dev_name, GFP_KERNEL);
 	if (!dev_name)
 		return -ENOMEM;
 
···
 		}
 	}
 
-	kfree(dev_name);
+	kfree(orig);
 	return 0;
 err:
 	bch2_darray_str_exit(ret);
-	kfree(dev_name);
+	kfree(orig);
 	return -ENOMEM;
 }