JFS: Improved error handing

JFS: Improved error handing

This patch replaces many assert statements, which caused a BUG(), with
improved code to mark the superblock dirty and then proceed as specified by
the errors= mount flag (as ext2 and ext3 do).  JFS's default for the errors
option is "remount-ro" in order to prevent addition data corruption when a
problem is found.

These asserts are usually triggered by on-disk data corruption.  By marking
the superblock dirty, fsck will perform a complete check on the file system
and correct the problems, rather than simply replaying the journal, inviting
later trouble.

Submitted by Karl Rister & Dave Kleikamp