xen-blkfront: pre-allocate pages for requests

This prevents us from having to call alloc_page while we are preparing
the request. Since blkfront was calling alloc_page with a spinlock
held we used GFP_ATOMIC, which can fail if we are requesting a lot of
pages since it is using the emergency memory pools.

Allocating all the pages at init prevents us from having to call
alloc_page, thus preventing possible failures.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xen.org
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>