btrfs: open-code btrfs_copy_from_user()

The function btrfs_copy_from_user() handles the folio dirtying for
buffered write. The original design is to allow that function to handle
multiple folios, but since commit c87c2997 ("btrfs: make buffered
write to copy one page a time") there is no need to support multiple
folios.

So here open-code btrfs_copy_from_user() to
copy_folio_from_iter_atomic() and flush_dcache_folio() calls.

The short-copy check and revert are still kept as-is.

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