debugfs: fix several errors and add assertions in the set_fields command

debugfs: fix several errors and add assertions in the set_fields command

Fix copy-n-paste errors:
* remove duplicate "lastcheck" and "min_extra_isize"
* fix pointer for "first_error_line" and "last_error_line"
* remove superblock field "inodes_count" from inode fields
* add null-termination for mmp_fields

Add assertions for catching such errors in the future.
Mark true aliases with flag "FLAG_ALIAS" and suppress assert for them.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>