ext4: provide ext4_issue_zeroout()

Create new function ext4_issue_zeroout() to zeroout contiguous (both
logically and physically) part of inode data. We will need to issue
zeroout when extent structure is not readily available and this function
will allow us to do it without making up fake extent structures.

Change-Id: I5deb04b49d3ebdd1ac12f8bb950faf46d08f5d80
Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Git-commit: 53085fac02d12fcd29a9cb074ec480ff0f77ae5c
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.4


[srkupp@codeaurora.org: Resolved minor conflict]
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>