prctl: remove one-shot limitation for changing exe link

This limitation came with the reason to remove "another way for
malicious code to obscure a compromised program and masquerade as a
benign process" by allowing "security-concious program can use this
prctl once during its early initialization to ensure the prctl cannot
later be abused for this purpose":

    http://marc.info/?l=linux-kernel&m=133160684517468&w=2

This explanation doesn't look sufficient.  The only thing "exe" link is
indicating is the file, used to execve, which is basically nothing and
not reliable immediately after process has returned from execve system
call.

Moreover, to use this feture, all the mappings to previous exe file have
to be unmapped and all the new exe file permissions must be satisfied.

Which means, that changing exe link is very similar to calling execve on
the binary.

The need to remove this limitations comes from migration of NFS mount
point, which is not accessible during restore and replaced by other file
sys...