LU-2642 osd-ldiskfs: fix typo in REQ_WRITE check

LU-2642 osd-ldiskfs: fix typo in REQ_WRITE check

Commit eecb3086 defines __REQ_WRITE as BIO_RW if __REQ_WRITE is
not defined. __REQ_WRITE is an enum. REQ_WRITE is a define.

Let's use that instead.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I500c771e6f3e69e1be3fab872150019103515d30
Reviewed-on: http://review.whamcloud.com/5503
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>