xfs: remove deprecated mount options

ikeep/noikeep was a workaround for old DMAPI code which is no longer
relevant.

attr2/noattr2 - is for controlling upgrade behaviour from fixed attribute
fork sizes in the inode (attr1) and dynamic attribute fork sizes (attr2).
mkfs has defaulted to setting attr2 since 2007, hence just about every
XFS filesystem out there in production right now uses attr2.

Signed-off-by: Pavel Reichl <preichl@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix minor typos]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>