userns: Ignore suid and sgid on binaries if the uid or gid can not be mapped

When performing an exec where the binary lives in one user namespace and
the execing process lives in another usre namespace there is the possibility
that the target uids can not be represented.

Instead of failing the exec simply ignore the suid/sgid bits and run
the binary with lower privileges.   We already do this in the case
of MNT_NOSUID so this should be a well tested code path.

As the user and group are not changed this should not introduce any
security issues.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>