f2fs: add a proc entry show inject stats

This patch adds a proc entry named inject_stats to show total injected
count for each fault type.

cat /proc/fs/f2fs/<dev>/inject_stats
fault_type              injected_count
kmalloc                 0
kvmalloc                0
page alloc              0
page get                0
alloc bio(obsolete)     0
alloc nid               0
orphan                  0
no more block           0
too big dir depth       0
evict_inode fail        0
truncate fail           0
read IO error           0
checkpoint error        0
discard error           0
write IO error          0
slab alloc              0
dquot initialize        0
lock_op                 0
invalid blkaddr         0
inconsistent blkaddr    0
no free segment         0
inconsistent footer     0

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>