NFSv4.1: Simplify slot allocation

Clean up the NFSv4.1 slot allocation by replacing nfs_find_slot() with
a function nfs_alloc_slot() that returns a pointer to the nfs4_slot
instead of an offset into the slot table.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>