xfs: compile out v4 support if disabled

xfs: compile out v4 support if disabled

Add a few strategic IS_ENABLED statements to let the compiler eliminate
unused code when CONFIG_XFS_SUPPORT_V4 is disabled.

This saves multiple kilobytes of .text in my .config:

$ size xfs.o.*
text	   data	    bss	    dec	    hex	filename
1363633	 294836	    592	1659061	 1950b5	xfs.o.new
1371453	 294868	    592	1666913	 196f61	xfs.o.old

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