udf: remove wrong prototype of udf_readdir

sparse generated:
fs/udf/dir.c:78:5: warning: symbol 'udf_readdir' was not declared. Should it be static?
there are 2 different prototypes of udf_readdir - remove them and move
code around to make it still compile

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>