f2fs : Set correct bio flags to reflect REQ_RAHEAD

In f2fs_grab_read_bio(), bio op_flag setting is not
handled properly resulting in request read-ahead
never set. This change corrects bio flag setting to
reflect REQ_RAHEAD if passed by caller.

Change-Id: I5d7b0045fac891bff37073b8d1343fd2aa7b08a5
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>