vfs: fix check for fallocate on active swapfile

Fix the broken check for calling sys_fallocate() on an active swapfile,
introduced by commit 0790b31b

 ("fs: disallow all fallocate
operation on active swapfile").

Signed-off-by: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>