nfs: simplify nfs_folio_find_and_lock_request

nfs_folio_find_and_lock_request and the nfs_page_group_lock_head helper
called by it spend quite some effort to deal with head vs subrequests.
But given that only the head request can be stashed in the folio private
data, non of that is required.

Fold the locking logic from nfs_page_group_lock_head into
nfs_folio_find_and_lock_request and simplify the result based on the
invariant that we always find the head request in the folio private data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>