mm/readahead.c: simplify get_next_ra_size()

mm/readahead.c: simplify get_next_ra_size()

It's a trivial simplification for get_next_ra_size() and clear enough for
humans to understand.

It also fixes potential overflow if ra->size(< ra_pages) is too large.

Link: http://lkml.kernel.org/r/1540707206-19649-1-git-send-email-hsiangkao@aol.com
Signed-off-by: Gao Xiang <hsiangkao@aol.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>