[XFS] fix XFS quota for modular XFS builds

Cannot build XFS filesystem support as module with quota support.  It
works only when the XFS filesystem support is compiled into the kernel.
Menuconfig prevents from setting CONFIG_XFS_FS=m and CONFIG_XFS_QUOTA=y.

How to reproduce: configure the XFS filesystem with quota support as
module.  The resulting kernel won't have quota support compiled into
xfs.ko.

Fix: Changing the fs/xfs/Kconfig file from tristate to bool lets you
configure the quota support to be compiled into the XFS module.  The
Makefile-linux-2.6 checks only for CONFIG_XFS_QUOTA=y.

Signed-off-by: Dimitri Puzin <tristan-777@ddkom-online.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Nathan Scott <nathans@sgi.com>