cleanup xfs_sb.h feature flag helpers

The various inlines in xfs_sb.h that deal with the superblock version
and fature flags were converted from macros a while ago, and this
show by the odd coding style full of useless braces and backslashes
and the avoidance of conditionals.

Clean these up to look like normal C code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>