Linux: Defer loading the object set in zfs_setattr()

We need to wait until after having done a zfs_enter() to load some
fields from the zfsvfs structure. Otherwise a use-after-free is
possible in the face of a concurrent rollback.

Other functions in this file are careful to avoid this bug, I believe
this is the only instance.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #15752