mm/readahead: make do_page_cache_ra take a readahead_control

mm/readahead: make do_page_cache_ra take a readahead_control

Rename __do_page_cache_readahead() to do_page_cache_ra() and call it
directly from ondemand_readahead() instead of indirecting via ra_submit().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Eric Biggers <ebiggers@google.com>
Link: https://lkml.kernel.org/r/20200903140844.14194-5-willy@infradead.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>