btrfs: use a single variable for return value at lookup_inline_extent_backref()

At lookup_inline_extent_backref(), instead of using a 'ret' and an 'err'
variable for tracking the return value, use a single one ('ret'). This
simplifies the code, makes it comply with most of the existing code and
it's less prone for logic errors as time has proven over and over in the
btrfs code.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>