afs: Fix leak in afs_lookup_cell_rcu()

Fix a leak on the cell refcount in afs_lookup_cell_rcu() due to
non-clearance of the default error in the case a NULL cell name is passed
and the workstation default cell is used.

Also put a bit at the end to make sure we don't leak a cell ref if we're
going to be returning an error.

This leak results in an assertion like the following when the kafs module is
unloaded:

	AFS: Assertion failed
	2 == 1 is false
	0x2 == 0x1 is false
	------------[ cut here ]------------
	kernel BUG at fs/afs/cell.c:770!
	...
	RIP: 0010:afs_manage_cells+0x220/0x42f [kafs]
	...
	 process_one_work+0x4c2/0x82c
	 ? pool_mayday_timeout+0x1e1/0x1e1
	 ? do_raw_spin_lock+0x134/0x175
	 worker_thread+0x336/0x4a6
	 ? rescuer_thread+0x4af/0x4af
	 kthread+0x1de/0x1ee
	 ? kthread_park+0xd4/0xd4
	 ret_from_fork+0x24/0x30

Fixes: 989782dc

 ("afs: Overhaul cell database management")
Signed-off-by: David Howells <dhowells@redhat.com>