selinux: always check the file label in selinux_kernel_read_file()

Commit 2039bda1 ("LSM: Add "contents" flag to kernel_read_file hook")
added a new flag to the security_kernel_read_file() LSM hook, "contents",
which was set if a file was being read in its entirety or if it was the
first chunk read in a multi-step process.  The SELinux LSM callback was
updated to only check against the file label if this "contents" flag was
set, meaning that in multi-step reads the file label was not considered
in the access control decision after the initial chunk.

Thankfully the only in-tree user that performs a multi-step read is the
"bcm-vk" driver and it is loading firmware, not a kernel module, so there
are no security regressions to worry about.  However, we still want to
ensure that the SELinux code does the right thing, and *always* checks
the file label, especially as there is a chance the file could change
between chunk reads.

Fixes: 2039bda1 ("LSM: Add "contents" flag to kernel_read_file hook")
Signed-off-by: Paul Moore <paul@paul-moore.com>