rbd-nbd: don't look for cmdline in /proc/[tid] subdirectories

rbd-nbd: don't look for cmdline in /proc/[tid] subdirectories

The proc fs contains both /proc/[pid] and /proc/[tid]
subdirectories. The /proc/[tid] subdirectories are not visible
when listing /proc. But when checking pid for reattached process,
get_mapped_info directly tries to open /proc/[pid]/cmdline, and
it may actually be tid belonging to some other rbd-nbd
process. Try to avoid this by checking /proc/[pid]/comm first --
for pid we expect to find "rbd-nbd" here.

Signed-off-by: Mykola Golub <mgolub@suse.com>