diag: Add protection while freeing diag mempool buffer

There is possibility of double free because of protection
is missing while freeing diag hdlc mempool buffer. The patch
adds protection for the same by taking spinlock.

Change-Id: I7d2176e05408df80c839e198994807663ce172e8
Signed-off-by: Hardik Arya <harya@codeaurora.org>