NFSD: Call nfsd4_init_slabs() from init_nfsd()

init_nfsd() was calling free_slabs() during cleanup code, but the call
to init_slabs() was hidden in nfsd4_state_init().  This could be
confusing to people unfamiliar with the code.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>