ixgbevf: break out Rx buffer page management

Based on commit e0142726 ("igb: Break out Rx buffer page management")

Consolidate Rx code paths to reduce duplication when we expand them in
the future.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>