man: list xfs_io lsattr inode flag letters

man: list xfs_io lsattr inode flag letters

The section of the xfs_io manpage for the 'chattr' command says to refer
to xfsctl(3) for information on the flags.  The inode flag information
was moved to ioctl_xfs_fssetxattr(2) ages ago, and it never actually
mapped the inode flag letters to inode flag bits, so fix the link and
add such a mapping to the xfs_io manpage.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>