x86/pkeys: Clarify PKRU_AD_KEY macro

When changing the PKRU_AD_KEY macro to be used for PKS the name came
into question.[1]

The intent of PKRU_AD_KEY is to set an initial value for the PKRU
register but that is just a mask value.

Clarify this by changing the name to PKRU_AD_MASK().

NOTE the checkpatch errors are ignored for the init_pkru_value to align
the values in the code.

[1] https://lore.kernel.org/lkml/eff862e2-bfaa-9e12-42b5-a12467d72a22@intel.com/



Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220419170649.1022246-3-ira.weiny@intel.com