bump grootfs
bump grootfs

[finishes #155822115]

Submodule src/code.cloudfoundry.org/grootfs 8ec2b48..c2b462b:
  > Handle stat errors when checking for mountpoint

Signed-off-by: Georgi Sabev <cd7643e50243a10153b49cb72b4bdbf8b48cb045@gmail.com>

cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release,cloudfoundry-incubator/guardian-release