mm: cma: fix stack corruption due to sprintf usage

name[] in cma_debugfs_add_one() can only accommodate 16 chars including
NULL to store sprintf output.  It's common for cma device name to be larger
than 15 chars. This can cause stack corrpution. If the gcc stack protector
is turned on, this can cause a panic due to stack corruption.

Below is one example trace:

Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in:
ffffff8e69a75730
Call trace:
  [<ffffff8e68289504>] dump_backtrace+0x0/0x2c4
  [<ffffff8e682897e8>] show_stack+0x20/0x28
  [<ffffff8e685ea808>] dump_stack+0xb8/0xf4
  [<ffffff8e683c454c>] panic+0x154/0x2b0
  [<ffffff8e682a724c>] print_tainted+0x0/0xc0
  [<ffffff8e69a75730>] cma_debugfs_init+0x274/0x290
  [<ffffff8e682839ec>] do_one_initcall+0x5c/0x168
  [<ffffff8e69a50e24>] kernel_init_freeable+0x1c8/0x280

Fix the short sprintf buffer in cma_debugfs_add_one() by using scnprintf()
instead of sprintf().

Change-Id: I627e7bc96d986ba5ece6e81adeaa2aac5c4a11d4
fixes: f318dd083c81 ("cma: Store a name in the cma structure")
Patch-mainline: linux-mm @ 11 Aug 2017 15:40:17
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>