xfs: clean up xfs_fsops.h

Use struct types instead of typedefs so that the header can be included
with pulling in the headers that define the typedefs, and remove the
pointless externs.

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>