Linux 4.11 compat: avoid refcount_t name conflict

Related to commit 4859fe7, 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 openzfs#7148

include/linux/vfs_compat.h

@@ -272,6 +272,10 @@ lseek_execute(
  * This is several orders of magnitude larger than expected grace period.
  * At 60 seconds the kernel will also begin issuing RCU stall warnings.
  */
+#ifdef refcount_t
+#undef refcount_t
+#endif
+
 #include <linux/posix_acl.h>
 
 #if defined(HAVE_POSIX_ACL_RELEASE) && !defined(HAVE_POSIX_ACL_RELEASE_GPL_ONLY)
@@ -397,6 +401,8 @@ typedef mode_t zpl_equivmode_t;
 #define	zpl_posix_acl_valid(ip, acl)  posix_acl_valid(acl)
 #endif
 
+#define	refcount_t	zfs_refcount_t
+
 #endif /* CONFIG_FS_POSIX_ACL */
 
 /*