Linux 4.11 compat: avoid refcount_t name conflict

Related to commit 4859fe796, when directly using the kernel's
refcount functions in kernel compatibility code do not map
refcount_t to zfs_refcount_t. This leads to a type mismatch.

Longer term we should consider renaming refcount_t to
zfs_refcount_t in the zfs code base.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7148