Change nfsv4_getipaddr() and nfsrpc_fillsa() to not use sockaddr_storage.

This patch changes nfsv4_getipaddr() and nfsrpc_fillsa() to use
a sockaddr_in * and sockaddr_in6 * instead of sockaddr_storage, to
avoid allocating the latter on the stack. It also moves the nfsrpc_fillsa()
call to after the completion of parsing of the DeviceInfo reply from
the server. This patch is in preparation for addition of Flex File
Layout support in a future commit.
It only affects the "pnfs" NFSv4.1 client mount option and should not
have changed its semantics.