evm: permit only valid security.evm xattrs to be updated
In addition to requiring CAP_SYS_ADMIN permission to modify/delete
security.evm, prohibit invalid security.evm xattrs from changing,
unless in fixmode. This patch prevents inadvertent 'fixing' of
security.evm to reflect offline modifications.
Changelog v7:
- rename boot paramater 'evm_mode' to 'evm'