NTB: ntb_transport: Use scnprintf() for avoiding potential buffer overflow

Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit.  Fix it by replacing with scnprintf().

Fixes: fce8a7bb (PCI-Express Non-Transparent Bridge Support)
Fixes: 282a2fee (NTB: Use DMA Engine to Transmit and Receive)
Fixes: a754a8fc (NTB: allocate number transport entries depending on size of ring size)
Fixes: d98ef99e (NTB: Clean up QP stats info)
Fixes: e74bfeed (NTB: Add flow control to the ntb_netdev)
Fixes: 569410ca (NTB: Use unique DMA channels for TX and RX)
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>