UBIFS: re-arrange debugging code a bit

Move 'dbg_debugfs_init()' and 'dbg_debugfs_exit()' functions which initialize
debugfs for whole UBIFS subsystem below the code which initializes debugfs for
a particular UBIFS instance. And do the same for 'ubifs_debugging_init()' and
'ubifs_debugging_exit()' functions. This layout is a bit better for the next
patches, so this is just a preparation.

Also, rename 'open_debugfs_file()' into 'dfs_file_open()' for consistency.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>