SUNRPC: move bkl locking and xdr proc invocation into a common helper

Since every invocation of xdr encode or decode functions takes the BKL now,
there's a lot of redundant lock_kernel/unlock_kernel pairs that we can pull
out into a common function.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>