afs: Fix merge preference rule failure condition

In the Linux kernel, the following vulnerability has been resolved:

afs: Fix merge preference rule failure condition

syzbot reported a lock held when returning to userspace[1]. This is

because if argc is less than 0 and the function returns directly, the held

inode lock is not released.

Fix this by store the error in ret and jump to done to clean up instead of

returning directly.

[dh: Modified Lizhi Xu's original patch to make it honour the error code

from afs_split_string()]

[1]

WARNING: lock held when returning to user space!

6.13.0-rc3-syzkaller-00209-g499551201b5f #0 Not tainted

------------------------------------------------

syz-executor133/5823 is leaving the kernel with locks still held!

1 lock held by syz-executor133/5823:

#0: ffff888071cffc00 (&sb->s_type->i_mutex_key#9){++++}-{4:4}, at: inode_lock include/linux/fs.h:818 [inline]

#0: ffff888071cffc00 (&sb->s_type->i_mutex_key#9){++++}-{4:4}, at: afs_proc_addr_prefs_write+0x2bb/0x14e0 fs/afs/addr_prefs.c:388