nilfs: Convert nilfs_set_page_dirty() to nilfs_dirty_folio()

The comment about the page always being locked is wrong, so copy
the locking protection from __set_page_dirty_buffers().  That
means moving the call to nilfs_set_file_dirty() down the
function so as to not acquire a new dependency between the
mapping->private_lock and the ns_inode_lock.  That might be a
harmless dependency to add, but it's not necessary.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Acked-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Tested-by: Mike Marshall <hubcap@omnibond.com> # orangefs
Tested-by: David Howells <dhowells@redhat.com> # afs