md/raid5-log: Drop extern decorators for function prototypes

extern is not necessary and recommended against when defining prototype
functions in headers. checkpatch.pl complains about these. So remove
them.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>