eal/linux: allow hugetlbfs sub-directories

eal/linux: allow hugetlbfs sub-directories

get_hugepage_dir() was implemented in such a way that a --huge-dir
option had to exactly match the mountpoint, but there's no reason for
this restriction: DPDK might not be the only user of hugepages, and
shouldn't assume it owns an entire mountpoint. For example, if I have
/dev/hugepages/myapp, and /dev/hugepages/dpdk, I should be able to
specify:

--huge-dir=/dev/hugepages/dpdk/

and have DPDK only use that sub-directory.

Fix the implementation to allow a sub-directory within a suitable
hugetlbfs mountpoint to be specified, preferring the closest match.

Signed-off-by: John Levon <john.levon@nutanix.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>