snapuserd: add a function to check if kernel supports ublk

UBLK is enabled in android-common-kernels 6.1 and above.

Bug: 414812023
Test: th
Change-Id: I9a5e7266c367a1a5245acee8632d229448976424
Signed-off-by: Sandeep Dhavale <dhavale@google.com>