mem/linux: fix hugedir write deadlock

mem/linux: fix hugedir write deadlock

At hugepage info initialization, EAL takes out a write lock on
hugetlbfs directories, and drops it after the memory init is
finished. However, in non-legacy mode, if "-m" or "--socket-mem"
switches are passed, this leads to a deadlock because EAL tries
to allocate pages (and thus take out a write lock on hugedir)
while still holding a separate hugedir write lock in EAL.

Fix it by checking if write lock in hugepage info is active, and
not trying to lock the directory if the hugedir fd is valid.

Fixes: 1a7dc2252f28 ("mem: revert to using flock and add per-segment lockfiles")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Shahaf Shuler <shahafs@mellanox.com>
Tested-by: Andrew Rybchenko <arybchenko@solarflare.com>