btrfs: remove pointless out labels from disk-io.c

btrfs: remove pointless out labels from disk-io.c

Some functions (btrfs_validate_extent_buffer() and
btrfs_start_pre_rw_mount()) have an 'out' label that does nothing but
return, making it pointless. Simplify this by removing the label and
returning instead of gotos plus setting the 'ret' variable.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>