powerpc/secvar: Warn when PAGE_SIZE is smaller than max object size

Due to sysfs constraints, when writing to a variable, we can only handle
writes of up to PAGE_SIZE.

It's possible that the maximum object size is larger than PAGE_SIZE, in
which case, print a warning on boot so that the user is aware.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-13-ajd@linux.ibm.com