xfs: fix issues that cause userspace warnings

Some of the code shared with userspace causes compilation warnings
from things turned off in the kernel code, such as differences in
variable signedness. Fix those issues.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>