btrfs: merge setting ret and return ret

btrfs: merge setting ret and return ret

In many places we have pattern:

	ret = ...;
	return ret;

This can be simplified to a direct return, removing 'ret' if not
otherwise needed. The places in self tests are not converted so we can
add more test cases without changing surrounding code
(extent-map-tests.c:test_case_4()).

Signed-off-by: David Sterba <dsterba@suse.com>