MdeModulePkg VariablePei: Return GetVariable() attr if EFI_BUFFER_TOO_SMALL

MdeModulePkg VariablePei: Return GetVariable() attr if EFI_BUFFER_TOO_SMALL

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2062

This commit makes the behavior for PeiGetVariable() match the following
specification-defined behavior. It is now consistent with the DXE/SMM
variable driver implementation.

The UEFI specification v2.8 Errata A Section 8.2 "GetVariable()"
"Attributes" parameter description states:

"If not NULL, a pointer to the memory location to return the
 attributes bitmask for the variable. See 'Related Definitions.'
 If not NULL, then Attributes is set on output both when
 EFI_SUCCESS and when EFI_BUFFER_TOO_SMALL is returned."

The attributes were previously only returned from the implementation
in Variable.c on EFI_SUCCESS. They are now returned on EFI_SUCCESS or
EFI_BUFFER_TOO_SMALL according to spec.

Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Guomin Jiang <guomin.jiang@intel.com>