fs/adfs: clean up error message printing

Overhaul our message printing:

- provide a consistent way to print messages:
  - filesystem corruption should be reported via adfs_error()
  - everything else should use adfs_msg()
- clean up the error message printing when mounting a filesystem
- fix the messages printed by the big directory format code to only
  use adfs_error() when there is filesystem corruption, otherwise
  use adfs_msg().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>