xfs: warn about inodes with project id of -1

Inodes aren't supposed to have a project id of -1U (aka 4294967295) but
the kernel hasn't always validated FSSETXATTR correctly.  Flag this as
something for the sysadmin to check out.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>