ext4: export msg_count and warning_count via sysfs

This numbers can be analized by system automation similar to errors_count.
In ideal world it would be nice to have separate counters for different
log-levels, but this makes this patch too intrusive.

Signed-off-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Link: https://lore.kernel.org/r/20200725123313.4467-1-dmtrmonakhov@yandex-team.ru


Signed-off-by: Theodore Ts'o <tytso@mit.edu>