jbd: Move debug message into #ifdef area

Move call to jbd_debug() into #ifdef CONFIG_JBD_DEBUG block because
'dropped' is declared there. The code could be compiled without this
change anyway, simply because jbd_debug() expands to nothing if
!CONFIG_JBD_DEBUG but IMHO it doesn't look good in general.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>