[PATCH 35/64] cachefiles: Add security derivation

[David Howells]

I missed out the patch description:

    cachefiles: Add security derivation
    
    Implement code to derive a new set of creds for the cachefiles to use when
    making VFS or I/O calls and to change the auditing info since the
    application interacting with the network filesystem is not accessing the
    cache directly.  Cachefiles uses override_creds() to change the effective
    creds temporarily.
    
    set_security_override_from_ctx() is called to derive the LSM 'label' that
    the cachefiles driver will act with.  set_create_files_as() is called to
    determine the LSM 'label' that will be applied to files and directories
    created in the cache.  These functions alter the new creds.
    
    Also implement a couple of functions to wrap the calls to begin/end cred
    overriding.
    
    Signed-off-by: David Howells <dhowells at redhat.com>
    cc: linux-cachefs at redhat.com

David