debugfs: properly set up extent header in do_write

debugfs: properly set up extent header in do_write

do_write doesn't fully set up the first extent header on a new
inode, so if we write a 0-length file, and don't write any data
to the new file, we end up creating something that looks corrupt
to kernelspace:

EXT4-fs error (device loop0): ext4_ext_check_inode:464: inode #12: comm ls: bad header/extent: invalid magic - magic 0, entries 0, max 0(0), depth 0(0)

Do something similar to ext4_ext_tree_init() here, and
fill out the first extent header upon creation to avoid this.

Reported-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Tested-by: Robert Yang <liezhi.yang@windriver.com>