nfs: Pass the file pointer to nfs_symlink_filler()

In preparation for unifying the read_cache_page() and read_folio()
implementations, make nfs_symlink_filler() get the inode
from the page instead of passing it in from read_cache_page().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>