buffer: hoist GFP flags from grow_dev_page() to __getblk_gfp()

grow_dev_page() is only called by grow_buffers().  grow_buffers() is only
called by __getblk_slow() and __getblk_slow() is only called from
__getblk_gfp(), so it is safe to move the GFP flags setting all the way
up.  With that done, add a new bdev_getblk() entry point that leaves the
GFP flags the way the caller specified them.

[willy@infradead.org: fix grow_dev_page() error handling]
  Link: https://lkml.kernel.org/r/ZRREEIwqiy5DijKB@casper.infradead.org
Link: https://lkml.kernel.org/r/20230914150011.843330-3-willy@infradead.org


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