ima,evm: move initcalls to the LSM framework

ima,evm: move initcalls to the LSM framework

This patch converts IMA and EVM to use the LSM frameworks's initcall
mechanism. It moved the integrity_fs_init() call to ima_fs_init() and
evm_init_secfs(), to work around the fact that there is no "integrity" LSM,
and introduced integrity_fs_fini() to remove the integrity directory, if
empty. Both integrity_fs_init() and integrity_fs_fini() support the
scenario of being called by both the IMA and EVM LSMs.

This patch does not touch any of the platform certificate code that
lives under the security/integrity/platform_certs directory as the
IMA/EVM developers would prefer to address that in a future patchset.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
[PM: adjust description as discussed over email]
Signed-off-by: Paul Moore <paul@paul-moore.com>