Currently there is no locking mechanism to protect global
variable ll_stats.result in hdd_debugfs_stats_update api and
wlan_hdd_llstats_free_buf api so if two threads access the file
simultaneously there is a possibility of use after free of the
llstats buffer.
To address this issue add a mutex to prevent the simultaneous
access of the llstats buffer.
Change-Id: I0fd418e3a2034f10ba45021af21920f5e133cb6e
CRs-Fixed: 2157283
2e01abcc20