ovs-atomic-pthreads: Fix "has incomplete type" error.

ovs-atomic-pthreads: Fix "has incomplete type" error.

Commit 97be153858b4cd175cbe7862b8e1624bf22ab98a (clang: Add
annotations for thread safety check.) defined 'struct ovs_mutex'
variable in 'atomic_flag' in 'ovs-atomic-pthreads.h'. This
casued "mutex: has incomplete type" error in compilation when
'ovs-atomic-pthreads.h' is included.

This commit goes back to use 'pthread_mutex_t' for that variable
and adds test for the 'atomic_flag' related functions.

Reported-by: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>