xfs: clean up the XFS_IOC_FSCOUNTS handler

Split XFS_IOC_FSCOUNTS out of the main xfs_file_ioctl function, and
merge the xfs_fs_counts helper into the ioctl handler.

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>