mem: track page fd in non-single file mode

mem: track page fd in non-single file mode

Previously, we were only tracking lock file fd's in single-file
segments mode, but did not track fd's in non-single file mode
because we didn't need to (mmap() call still kept the lock). Now
that we are going to expose these fd's to the world, we need to
have access to them, so track them even in non-single file
segments mode.

We don't need to close fd's after mmap() because we're still
tracking them in an fd list. Also, for anonymous hugepages mode,
fd will always be -1 so exit early on error.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>