md/raid5: separate large if clause out of fetch_block().
md/raid5: separate large if clause out of fetch_block().

fetch_block() has a very large and hard to read 'if' condition.

Separate it into its own function so that it can be
made more readable.

Signed-off-by: NeilBrown <1c00aa603bb92d594c03cff60ecf8e63b2a5bcb1@suse.de>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs