V2 fields are set on a V1 journal superblock, or an
internal V2 journal has s_nr_users is non-zero, clear the
entire journal superblock beyond the V1 superblock. This
fixes botched V1->V2 updates.
problem.c, problem.h (PR_0_CLEAR_V2_JOURNAL): Add new problem code.
f_bad_local_jnl: New test which tests for a V2 journal with bad
fields caused by a botched V1->V2 upgrade.
Theodore Ts'o