fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl

Add a root-only variant of the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl which
removes all users' claims of the key, not just the current user's claim.
I.e., it always removes the key itself, no matter how many users have
added it.

This is useful for forcing a directory to be locked, without having to
figure out which user ID(s) the key was added under.  This is planned to
be used by a command like 'sudo fscrypt lock DIR --all-users' in the
fscrypt userspace tool (http://github.com/google/fscrypt

).

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>