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

Reported-by:  <syzbot+76f33569875eb708e575@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=76f33569875eb708e575


Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241226012616.2348907-1-lizhi.xu@windriver.com/
Link: https://lore.kernel.org/r/529850.1736261552@warthog.procyon.org.uk


Tested-by:  <syzbot+76f33569875eb708e575@syzkaller.appspotmail.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>