diag: Do not open diag node for same client again

Same diag client is added again to diag client map
due to not checking client pid in current client map
table. This is resulting in having same client twice
in diag client map table. The patch fixes this issue
by checking diag client table before adding new client.

Change-Id: Iaf2e8324f0f7753193dc9f7a87cb12ae6c83f26b
Signed-off-by: Hardik Arya <harya@codeaurora.org>