afs: add missing up_write() on return

If afs_cell_alloc() fails, afs_cells_sem doesn't get unlocked, which
leads to a deadlock.  Unlock it before returning.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>