xfs: enforce one namespace per attribute

xfs: enforce one namespace per attribute

Source kernel commit: ea0b3e814741fb64e7785b564ea619578058e0b0

Create a standardized helper function to enforce one namespace bit per
extended attribute, and refactor all the open-coded hweight logic.  This
function is not a static inline to avoid porting hassles in userspace.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>