ANDROID: f2fs: check nr_pages for readahead

Old kernel versions have the issue, since upstream f2fs removed this flow by:
commit d4384de9eb5e ("f2fs: Remove readahead collision detection").

Bug: 206148707
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Fixes: 8c95605c41a1 ("f2fs: avoid readahead race condition")
Change-Id: I3a95f0b436d0a5ee420f41d892401a8d79d4fe79