buffer: Convert __block_write_begin() to take a folio

Almost all callers have a folio now, so change __block_write_begin()
to take a folio and remove 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>
12 files changed