btrfs: unify extent buffer allocation api

Make the extent buffer allocation interface consistent.  Cloned eb will
set a valid fs_info.  For dummy eb, we can drop the length parameter and
set it from fs_info.

The built-in sanity checks may pass a NULL fs_info that's queried for
nodesize, but we know it's 4096.

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

diff --cc fs/btrfs/ctree.c
Simple merge

diff --cc fs/btrfs/extent_io.c
Simple merge

diff --cc fs/btrfs/extent_io.h
Simple merge

diff --cc fs/btrfs/tests/extent-buffer-tests.c
Simple merge

diff --cc fs/btrfs/tests/inode-tests.c
Simple merge

diff --cc fs/btrfs/tests/qgroup-tests.c
Simple merge