mm/readahead: add DEFINE_READAHEAD

Patch series "Readahead patches for 5.9/5.10".

These are infrastructure for both the THP patchset and for the fscache
rewrite,

For both pieces of infrastructure being build on top of this patchset, we
want the ractl to be available higher in the call-stack.

For David's work, he wants to add the 'critical page' to the ractl so that
he knows which page NEEDS to be brought in from storage, and which ones
are nice-to-have.  We might want something similar in block storage too.
It used to be simple -- the first page was the critical one, but then mmap
added fault-around and so for that usecase, the middle page is the
critical one.  Anyway, I don't have any code to show that yet, we just
know that the lowest point in the callchain where we have that information
is do_sync_mmap_readahead() and so the ractl needs to start its life
there.

For THP, we havew the code that needs it.  It's actually the apex patch to
the series; the one which finally starts to allocate THPs and present them
to consenting filesystems:
http://git.infradead.org/users/willy/pagecache.git/commitdiff/798bcf30ab2eff278caad03a9edca74d2f8ae760



This patch (of 8):

Allow for a more concise definition of a struct readahead_control.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Eric Biggers <ebiggers@google.com>
Cc: David Howells <dhowells@redhat.com>
Link: https://lkml.kernel.org/r/20200903140844.14194-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20200903140844.14194-3-willy@infradead.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>