MAINTAINERS: update IOMAP FILESYSTEM LIBRARY and XFS FILESYSTEM

In IOMAP FILESYSTEM LIBRARY and XFS FILESYSTEM, the M(ail): entry is
redundant with the L(ist): entry, remove the redundant M(ail): entry.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>