buffer: make folio_create_empty_buffers() return a buffer_head

buffer: make folio_create_empty_buffers() return a buffer_head

Patch series "Finish the create_empty_buffers() transition", v2.

Pankaj recently added folio_create_empty_buffers() as the folio equivalent
to create_empty_buffers().  This patch set finishes the conversion by
first converting all remaining filesystems to call
folio_create_empty_buffers(), then renaming it back to
create_empty_buffers().  I took the opportunity to make a few
simplifications like making folio_create_empty_buffers() return the head
buffer and extracting get_nth_bh() from nilfs2.

A few of the patches in this series aren't directly related to
create_empty_buffers(), but I saw them while I was working on this and
thought they'd be easy enough to add to this series.  Compile-tested only,
other than ext4.

This patch (of 26):

Almost all callers want to know the first BH that was allocated for this
folio.  We already have that handy, so return it.

Link: https://lkml.kernel.org/r/20231016201114.1928083-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20231016201114.1928083-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>