uselib() is the legacy systemcall for loading shared libraries.
Nowadays, applications use dlopen() to load shared libraries, completely
implemented in user space via mmap().

For example, glibc uses MAP_COPY to mmap shared libraries. While this
maps to MAP_PRIVATE | MAP_DENYWRITE on Linux, Linux ignores any
MAP_DENYWRITE specification from user space in mmap.

With this change, all remaining in-tree users of MAP_DENYWRITE use it
to map an executable. We will be able to open shared libraries loaded
via uselib() writable, just as we already can via dlopen() from user
space.

This is one step into the direction of removing MAP_DENYWRITE from the
kernel. This can be considered a minor user space visible change.
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: David Hildenbrand <david@redhat.com>