LU-6995 osd: fix invalid use of bvec_iter_page

LU-6995 osd: fix invalid use of bvec_iter_page

bvec_iter_page is expecting a biovecs list as a first argument, not a
biovec. That works as long as there is only one biovec in the bio. If
there is more than one, then invalid memory is dereferences since bvl
is a single vector, not a list. This only affect 3.14+ kernels, where
HAVE_BVEC_ITER is true. Don't use bvec_iter_page, but create the right
macro to simply return the vector's bv_page. Bug introduced in commit
833b670a.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: I390b527bacae9dd650814bb19ab2dbc9184a605d
Reviewed-on: http://review.whamcloud.com/15967
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>