e2fsprogs: properly set up extent header in do_write

e2fsprogs: 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.

[YOCTO #3848]

(From OE-Core rev: 7d1e51681d25f6e6d2c20744825723ad5c83861c)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>