xfs_io: clarify -c in bmap documentation

xfs_io: clarify -c in bmap documentation

The bmap -c parameter displays the cow fork information for a file if
the kernel was built with CONFIG_XFS_DEBUG=y.  Since xfs_bmap doesn't
support it and it doesn't work generally, remove it from the manpages.
However, xfstests relies on the -c command to be documented in the help
screen so leave it there with a warning about its use.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>