ext4: Fix fallocate to update the file size in each transaction

ext4_fallocate needs to update file size in each transaction.  Otherwise
if we crash the file size won't be seen.  We were also not marking
the inode dirty after updating file size before.  Also when we try to
retry allocation due to ENOSPC, make sure we reset the variable ret so
that we actually do a retry.

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>