Linux 6.8 compat: handle mnt_idmap user_namespace change

struct mnt_idmap no longer has a struct user_namespace within it. Work
around this by creating a temporary with the copy of the map we need
taken from the idmap.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Youzhong Yang <yyang@mathworks.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15805