eCryptfs: delay writing 0's after llseek until write

Delay writing 0's out in eCryptfs after a seek past the end of the file
until data is actually written.

http://www.opengroup.org/onlinepubs/009695399/functions/lseek.html

``The lseek() function shall not, by itself, extend the size of a
file.''

Without this fix, applications that lseek() past the end of the file without
writing will experience unexpected behavior.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>