Fix tst-pkey.c pkey_alloc return checks and manual

This test was failing in some powerpc systems as it was not checking
for ENOSPC return.

As said on the Linux man-pages and can be observed by the implementation
at mm/mprotect.c in the Linux Kernel source.  The syscall pkey_alloc can
return EINVAL or ENOSPC.  ENOSPC will indicate either that all keys are
in use or that the kernel does not support pkeys.

Reviewed-by: Gabriel F. T. Gomes <gabriel@inconstante.net.br>