diag: Add mutex protection while reading dci debug statistics

Unserialized access to diag_dbgfs_dci_data_index can lead to
heap overflow. Add mutex protection while updating the
diag_dbgfs_dci_data_index.

Change-Id: Iee9d0447494e3576e6293afcd4d7611bc429aa8a
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>