buffer: Convert block_write_end() to take a folio

buffer: Convert block_write_end() to take a folio

All callers now have a folio, so pass it in instead of converting
from a folio to a page and back to a folio again.  Saves a call
to compound_head().

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>