buffer: add folio_alloc_buffers() helper

buffer: add folio_alloc_buffers() helper

Folio version of alloc_page_buffers() helper.  This is required to convert
create_page_buffers() to folio_create_buffers() later in the series.

alloc_page_buffers() has been modified to call folio_alloc_buffers() which
adds one call to compound_head() but folio_alloc_buffers() removes one
call to compound_head() compared to the existing alloc_page_buffers()
implementation.

Link: https://lkml.kernel.org/r/20230417123618.22094-3-p.raghav@samsung.com
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>