[SCSI] tgt: fix the user/kernel ring buffer interface

This patches fixes two bugs in the scsi target infrastructure's
user/kernel interface.

- It wrongly assumes that the ring buffer size of the interface (64KB)
is larger than or equal to the system page size. This patch sets the
ring buffer size to PAGE_SIZE if the system page size is larger.

- It uses PAGE_SIZE in the header file exported to userspace. This
patch removes it.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>