xfs: ensure log flush at the end of a synchronous fallocate call

Since we've started treating fallocate more like a file write, we
should flush the log to disk if the user has asked for synchronous
writes either by setting it via fcntl flags, or inode flags, or with
the sync mount option.  We've already got a helper for this, so use
it.

[The original patch by Darrick was massaged by Dave to fit this patchset]

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>