fuse2fs: delegate access control decisions to the kernel

fuse2fs: delegate access control decisions to the kernel

In "kernel" mode (aka allow_others + default_permissions), the kernel
enforces all the access control for us.  Therefore, we don't need to do
any checking of our own.  Create a purpose-built helper to detect this
situation and turn off all the access controlling.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/174553065373.1161102.14873909355987419902.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>