UPSTREAM: eCryptfs: Call lower ->flush() from ecryptfs_flush()

UPSTREAM: eCryptfs: Call lower ->flush() from ecryptfs_flush()

Since eCryptfs only calls fput() on the lower file in
ecryptfs_release(), eCryptfs should call the lower filesystem's
->flush() from ecryptfs_flush().

If the lower filesystem implements ->flush(), then eCryptfs should try
to flush out any dirty pages prior to calling the lower ->flush(). If
the lower filesystem does not implement ->flush(), then eCryptfs has no
need to do anything in ecryptfs_flush() since dirty pages are now
written out to the lower filesystem in ecryptfs_release().

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
BUG=chromium-os:35732
TEST=daisy build, ENOSPC bubbles up to userspace

(cherry picked from upstream commit 64e6651dcc10e9d2cc6230208a8e6c2cfd19ae18)
Signed-off-by: Kees Cook <keescook@chromium.org>
Change-Id: I44d6ba0eb30b6eebbfbedf3a1d7a8d7eaed28c01
Reviewed-on: https://gerrit.chromium.org/gerrit/44157
Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Will Drewry <wad@chromium.org>
Commit-Queue: Kees Cook <keescook@chromium.org>