MAINTAINERS: add Leah as xfs maintainer for 5.15.y

Update MAINTAINERS for xfs in an effort to help direct bots/questions
about xfs in 5.15.y.

Note: 5.10.y and 5.4.y will have different updates to their
respective MAINTAINERS files for this effort.

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>