NTB: ntb_transport: Use seq_file for QP stats debugfs

NTB: ntb_transport: Use seq_file for QP stats debugfs

The ./qp*/stats debugfs file for each NTB transport QP is currently
implemented with a hand-crafted kmalloc() buffer and a series of
scnprintf() calls. This is a pre-seq_file style pattern and makes future
extensions easy to truncate.

Convert the stats file to use the seq_file helpers via
DEFINE_SHOW_ATTRIBUTE(), which simplifies the code and lets the seq_file
core handle buffering and partial reads.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Jon Mason <jdmason@kudzu.us>