diag: Copy the message mask into buffer with correct length

In diag_msg_mask_copy copying the msg mask into destination buffer
with incorrect length can lead to out of bounds memory access for
the src buffer. Use correct length while copying the mask into the
destination buffer.

Change-Id: If7bcfa7c578e60ffc26af10046df5d1cbd85892d
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>