sdcardfs: Add gid and mask to private mount data

Adds support for mount2, remount2, and the functions
to allocate/clone/copy the private data

The next patch will switch over to actually using it.

Change-Id: I8a43da26021d33401f655f0b2784ead161c575e3
Signed-off-by: Daniel Rosenberg <drosen@google.com>