Add basic monitoring and status

[tim77]

Would be nice to add some basic monitoring and status how much ram compressed and compression ratio.

[hakavlad]

What it can monitor:

enabled                    Y
max_pool_percent           60
compressor                 lzo
zpool                      z3fold
same_filled_pages_enabled  Y
accept_threhsold_percent   50

same_filled_pages       0
stored_pages            0
pool_total_size         0
duplicate_entry         0
written_back_pages      0
reject_compress_poor    0
reject_kmemcache_fail   0
reject_alloc_fail       0
reject_reclaim_fail     0
pool_limit_hit          0

[hakavlad]

# zswap-cli --stats
Duplicate entries count: 0.
Pool limit hit: 0.
Pool total size: 0.
Reject allocation failures: 0.
Reject compression poor: 0.
Reject Kmemcache failures: 0.
Reject reclaim failures: 0.
Same filled pages count: 0.
Stored pages count: 0.
Written back pages count: 0.

This is not what I need.

[hakavlad]

I'd like to see:

enabled                    Y
max_pool_percent           60
compressor                 lzo
zpool                      z3fold
same_filled_pages_enabled  Y
accept_threhsold_percent   50

[hakavlad]

And:
pool_total_size in MiBs
stored_pages in MiBs

[hakavlad]

And
pool_total_size / MemTotal

[hakavlad]

And pool_total_size / stored_pages

[hakavlad]

Note that pool_total_size is in bytes.

[hakavlad]

pool_total_size: 300 MiB (5% MemTotal)
stored_pages: 600 MiB
Compression ratio: 2.0