ext4 crypto: allocate the right amount of memory for the on-disk symlink

Previously we were taking the required padding when allocating space
for the on-disk symlink.  This caused a buffer overrun which could
trigger a krenel crash when running fsstress.

Change-Id: I4e05ff207748192036de58bc5af91ae4c357b5b4
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>