diag: Increase itemsize of diag memory pool.

Diag allocates memory objects from mempool for copying packet from user
space to kernel space. Fix increases oject size of mempool to 4KB and any
packet of size greater than 4KB is dropped.

CRs-Fixed: 326579
Change-Id: I1cacd294df9a535fbe5f0466603eb7e69f7b43ef
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>