[XFS] Current usage of buftarg flags is incorrect.

The {test,set,clear}_bit() operations take a bit index for the bit to
operate on. The XBT_* flags are defined as bit fields which is incorrect,
not to mention the way the bit fields are enumerated is broken too. This
was only working by chance.

Fix the definitions of the flags and make the code using them use the
{test,set,clear}_bit() operations correctly.

SGI-PV: 958639
SGI-Modid: xfs-linux-melb:xfs-kern:27565a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>