exec: Check for errors when mapping the shared object

In the non-ASLR case, there is no check for an error from
vm_map_fixed(). Restore it, it was dropped in commit 939f0b6323e0a.
This bug could result in a refcount leak of the object used to map the
VDSO page.

Reviewed by: kib
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 1 week
Fixes: 939f0b6323e0 ("Implement shared page address randomization")
Differential Revision: https://reviews.freebsd.org/D53065

(cherry picked from commit 7117c86fcb7ebee4483097cefa9b143de8d78ee0)