buffer: use bdev_getblk() to avoid memory reclaim in readahead path

buffer: use bdev_getblk() to avoid memory reclaim in readahead path

__getblk() adds __GFP_NOFAIL, which is unnecessary for readahead; we're
quite comfortable with the possibility that we may not get a bh back.
Switch to bdev_getblk() which does not include __GFP_NOFAIL.

Link: https://lkml.kernel.org/r/20230914150011.843330-5-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>