ocfs2: fix NULL pointer dereference in __ocfs2_change_file_space()

As ocfs2_fallocate() will invoke __ocfs2_change_file_space() with a NULL
as the first parameter (file), it may trigger a NULL pointer dereferrence
due to a missing check.

Addresses http://bugs.launchpad.net/bugs/1006012

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Reported-by: Bret Towe <magnade@gmail.com>
Tested-by: Bret Towe <magnade@gmail.com>
Cc: Sunil Mushran <sunil.mushran@oracle.com>
Acked-by: Joel Becker <jlbec@evilplan.org>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>