Do not lock the volume when the volume maps to a file instead of a physical disk.

Do not lock the volume when the volume maps to a file instead of a physical disk.
Otherwise WriteBlocks() on the volume which maps to a file will fail.

Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14588 6f19259b-4bc3-4df7-8a09-765794883524