security: imbed evm calls in security hooks

Imbed the evm calls evm_inode_setxattr(), evm_inode_post_setxattr(),
evm_inode_removexattr() in the security hooks.  evm_inode_setxattr()
protects security.evm xattr.  evm_inode_post_setxattr() and
evm_inode_removexattr() updates the hmac associated with an inode.

(Assumes an LSM module protects the setting/removing of xattr.)

Changelog:
  - Don't define evm_verifyxattr(), unless CONFIG_INTEGRITY is enabled.
  - xattr_name is a 'const', value is 'void *'

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>