BACKPORT: mm/readahead: add folio allocation helper

Just a wrapper around filemap_alloc_folio() for now, but add it in
preparation for modifying the folio based on the 'ractl' being passed in.

No functional changes in this patch.

Bug: 416315033
(cherry picked from commit 1963de79)
[ akailash: Fix minor merge conflict retaining vendor hook
in page_cache_ra_unbounded() function. ]
Link: https://lkml.kernel.org/r/20241220154831.1086649-4-axboe@kernel.dk


Change-Id: I5e1c51ef8dad3c0b4f6b32ef240f9fa12ced8738
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Brian Foster <bfoster@redhat.com>
Cc: Chris Mason <clm@meta.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Akilesh Kailash <akailash@google.com>