udf: fix sparse warnings (shadowing & mismatch between declaration and definition)

fix sparse warnings:
fs/udf/super.c:1431:24: warning: symbol 'bh' shadows an earlier one
fs/udf/super.c:1347:21: originally declared here
fs/udf/super.c:472:6: warning: symbol 'udf_write_super' was not declared. Should it be static?

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Ben Fennema <bfennema@falcon.csc.calpoly.edu>
Cc: Jan Kara <jack@suse.cz>
Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>