context count of ssize_t versus int. Change context count to be ssize_t.

SGI-PV: 983395

SGI-Modid: xfs-linux-melb:xfs-kern:31333a
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>