BACKPORT: BACKPORT: uio: fix potential use after free issue when accessing debug_buffer

The variable debug_buffer is a global variable which is allocated
and free'd when open/close is called on debugfs file -
"/sys/kernel/debug/rmt_storage/info". The current code doesn't
have locks to handle concurrent accesses to the above file.
This results into use after free issue when debug_buffer is
accessed by two threads at the same time. Fix this by adding
a mutex lock to protect this global variable.

Change-Id: I6bc3f0ae2d7fca3ca9fe8561612f5863b6c3268a
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Bug: 33845464