ext4: add sysfs entry showing whether the fs contains errors

Currently there is no easy way to tell that the mounted file system
contains errors other than checking for log messages, or reading the
information directly from superblock.

This patch adds new sysfs entries:

errors_count		(number of fs errors we encounter)
first_error_time	(unix timestamp for the first error we see)
last_error_time		(unix timestamp for the last error we see)

If the file system is not marked as containing errors then any of the
file will return 0. Otherwise it will contain valid information. More
details about the errors should as always be found in the logs.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>