Linux 4.11 compat: avoid refcount_t name conflict

Linux 4.11 introduces a new type, refcount_t, which conflicts with the
type of the same name defined within ZFS.

Rename the ZFS type zfs_refcount_t. Within the ZFS code, use a macro to
cause references to refcount_t to be changed to zfs_refcount_t at
compile time. This reduces conflicts when later landing OpenZFS
patches.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #5823
Closes #5842