btrfs: convert btrfs_buffered_write() to use folios

The buffered write path is still heavily utilizing the page interface.
Since we have converted it to do a page-by-page copying, it's much easier
to convert all involved functions to folio interface, this involves:

- btrfs_copy_from_user()
- btrfs_drop_folio()
- prepare_uptodate_page()
- prepare_one_page()
- lock_and_cleanup_extent_if_need()
- btrfs_dirty_page()

All function are changed to accept a folio parameter, and if the word
"page" is in the function name, change that to "folio" too.

The function btrfs_dirty_page() is exported for v1 space cache, convert
v1 cache call site to convert its page to folio for the new interface.

And there is a small enhancement for prepare_one_folio(), instead of
manually waiting for the page writeback, let __filemap_get_folio() to
handle that by using FGP_WRITEBEGIN, which implies
(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE).

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>