ANDROID: Add filp_open_block() for zram

We currently plan to disallow use of filp_open() from drivers in GKI,
however the ZRAM driver still needs it. Add a new GKI-only variant of
filp_open() which only permits a block device to be opened, which can
be exported instead. This keeps ZRAM working but cuts down on drivers
that attempt to open and write files in kernel mode.

Bug: 179220339
Bug: 205141088
Change-Id: Id696b4aaf204b0499ce0a1b6416648670236e570
Signed-off-by: Alistair Delva <adelva@google.com>