scripts/gdb: add mount point list command

scripts/gdb: add mount point list command

lx-mounts will identify current mount points based on the 'init_task'
namespace by default, as we do not yet have a kernel thread list
implementation to select the current running thread.

Optionally, a user can specify a PID to list from that process'
namespace

Link: http://lkml.kernel.org/r/e614c7bc32d2350b4ff1627ec761a7148e65bfe6.1462865983.git.jan.kiszka@siemens.com
Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>