fio_plugin: fix hang in FIO

This is fix for https://github.com/spdk/spdk/issues/523



Fio hangs on pthread_exit(NULL) from spdk thread.
This happens because, pthread_exit tries to dlopen glibc and hangs on
__lll_lock_wait. This patch prevents unmapping of glibc in fio_plugin
and phtread_exit does not need to dlopen it again.

Signed-off-by: Piotr Pelplinski <piotr.pelplinski@intel.com>
Change-Id: I5078cc55e24841675d6ef4ecba43879dc3f73a4f

Reviewed-on: https://review.gerrithub.io/c/443912


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>