diag: Fix for diag debugfs buffer overflow

Diag debugfs buffer has potential buffer overflow scenario which can cause
memory corruption. Added safeguard to prevent this.

Crs-fixed: 585147
Change-Id: Ie1f099bb4bb626adff99ae225966aef70c1bc15e
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>