fix #2402: allow 1GB hugepages if 2MB is unavailable

fix #2402: allow 1GB hugepages if 2MB is unavailable

As reported in bug #2402, a system started with "default_hugepagesz=1G
hugepagesz=1G" does not have a /sys/kernel/mm/hugepages/hugepages-2048kB
directory.

To fix, ignore the missing directory in hugepages_mount (since it might
not be needed anyway), and correctly check if the requested hugepage
size is available in hugepages_size instead.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>