Make the buf_to_pages_noslab function available to the rest of the NFS
code. Rename it to nfs4_buf_to_pages_noslab to be consistent.

This will be used later in the NFSv4.2 xattr code.
Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>