Need a long hold in zpl_mount_impl

In zpl_mount_impl, there is:

    dmu_objset_hold	; returns with pool & ds held
    dsl_pool_rele

    sget

    dsl_dataset_rele

As spelled out in the "DSL Pool Configuration Lock" in dsl_pool.c,
this requires a long hold.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: John Poduska <jpoduska@datto.com>
Closes #10936