btrfs: reduce nesting and deduplicate error handling at btrfs_iget_path()

Make btrfs_iget_path() simpler and easier to read by avoiding nesting of
if-then-else statements and having an error label to do all the error
handling instead of repeating it a couple times.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>