Fix exec_mmap() to release the MM while we still have it active,

Fix exec_mmap() to release the MM while we still have it active,
to properly de-activate it and make the child_tid logic work
correctly.

Clear %fs/%gs in deactivate_mm() on x86, since our LDT will no
longer be valid after this.

Update mm_release() to deactivate MM state before releasing,
and avoid the expensive child_tid FUTEX if we're the last user
of the MM.