diag: Protect md_info structure while reallocation

The possibility of md_info structure being accessed simultaneously
by two threads is prevented by synchronizing while buffer
reallocation for hdlc encoding. Extend the scope of protection to
md_info till queueing the write to sdcard.

CRs-Fixed: 2279473
Change-Id: I75ddb102adfc6c79f35ed69914c9140cb82894c9
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>