ext4: Get journal write access before modifying the extent tree

When the user was writing into an unitialized extent,
ext4_ext_convert_to_initialize() was not requesting journal write access
before it started to modify the extent tree.   Fix this oversight.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>