f2fs: add proc entry to show discard_plist info

This patch adds a new proc entry to show discard_plist
information in more detail, which is very helpful to
know the discard pend list count clearly.

Such as:

Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
  0       390     156      85      67      46      37      26      14
  8        17      12       9       9       6      12      11      10
  16        5       9       2       4       8       3       4       1
  24        3       2       2       5       2       4       5       4
  32        3       3       2       3       .       3       3       1
  40        .       4       1       3       2       1       2       1
  48        1       .       1       1       .       1       1       .
  56        .       1       1       1       .       2       .       1
  64        1       2       .       .       .       .       .       .
  72        .       1       .       .       .       .       .       .
  80        3       1       .       .       1       1       .       .
  88        1       .       .       .       1       .       .       1
......

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>