When Xen is enabled, using BIOVEC_PHYS_MERGEABLE in a module
causes xen_biovec_phys_mergeable to be referenced, so it needs
to be exported.
Acked-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>

Impact: allow Xen control of bio merging

When running in Xen domain with device access, we need to make sure
the block subsystem doesn't merge requests across pages which aren't
machine physically contiguous.  To do this, we define our own
BIOVEC_PHYS_MERGEABLE.  When CONFIG_XEN isn't enabled, or we're not
running in a Xen domain, this has identical behaviour to the normal
implementation.  When running under Xen, we also make sure the
underlying machine pages are the same or adjacent.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>