UBIFS: amend debugging name check function prototype

Add 'struct ubifs_info *c' parameter to the 'dbg_check_name()' debugging
function - it will be needed in one of the following commits where we switch to
debugfs. So this is just a preparation.

Mark parameters as 'const' while on it.

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