Fix data race caused by open_fd_count

Wrap all open_fd_count accesses with atomic operators.

Signed-off-by: Ming Chen <v.mingchen@gmail.com>
Fix a trivial data race in xdr_nfs_argop4()

slhd is no more than a write scratch buffer.  It does not hurt much to make it
non-static, but this make tsan happier.

Signed-off-by: Ming Chen <v.mingchen@gmail.com>
Fix a trivial data race in put_gsh_client()

Signed-off-by: Ming Chen <v.mingchen@gmail.com>