MAINTAINERS: add Amir as xfs maintainer for 5.10.y

This is an attempt to direct the bots and human that are testing
LTS 5.10.y towards the maintainer of xfs in the 5.10.y tree.

This is not an upstream MAINTAINERS entry and 5.15.y and 5.4.y will
have their own LTS xfs maintainer entries.

Update Darrick's email address from upstream and add Amir as xfs
maintaier for the 5.10.y tree.

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/linux-xfs/Yrx6%2F0UmYyuBPjEr@magnolia/


Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>