xfs: don't expect xfs headers to be in subdirectories

Fix up some #include directives in preparation for moving a few
header files out of xfs source subdirectories.

Note that "xfs_linux.h" also got its quoting convention for included
files switched.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>