btrfs: Refactor the code that read from disk

btrfs: Refactor the code that read from disk

Move the code in charge to read the data from disk into a separate
function. This helps to separate the error handling logic (which
depends on the different raid profiles) from the read from disk
logic. Refactoring this code increases the general readability too.

This is a preparatory patch, to help the adding of the RAID 5/6 recovery code.

Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>