buffer: convert getblk_unmovable() and __getblk() to use bdev_getblk()

Move these two functions up in the file for the benefit of the next patch,
and pass in all of the GFP flags to use instead of the partial GFP flags
used by __getblk_gfp().

Link: https://lkml.kernel.org/r/20230914150011.843330-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Hui Zhu <teawater@antgroup.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>