Linux 4.19-rc3+ compat: Remove refcount_t compat

torvalds/linux@59b57717f ("blkcg: delay blkg destruction until
after writeback has finished") added a refcount_t to the blkcg
structure. Due to the refcount_t compatibility code, zfs_refcount_t
was used by mistake.

Resolve this by removing the compatibility code and replacing the
occurrences of refcount_t with zfs_refcount_t.

Reviewed-by: Franz Pletz <fpletz@fnordicwalking.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Closes #7885 
Closes #7932

include/linux/vfs_compat.h

@@ -297,9 +297,6 @@ 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>
 
@@ -430,8 +427,6 @@ 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 */
 
 /*