nfs: Convert nfs_symlink() to use a folio

Use the folio APIs, saving about four calls to compound_head().
Convert back to a page in each of the individual protocol implementations.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>