ext4: ENOSPC error handling for writing to an uninitialized extent

This patch handles possible ENOSPC errors when writing to an
uninitialized extent in case the filesystem is full.

A write to a prealloc area causes the split of an unititalized extent
into initialized and uninitialized extents.  If we don't have
space to add new extent information, instead of returning error,
convert the existing uninitialized extent to initialized one.  We
need to zero out the blocks corresponding to the entire extent to
prevent uninitialized data reaching userspace.

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>