Give strlcat() full buffer lengths rather than smaller buffer lengths

strlcat() is supposed to be given the length of the destination buffer,
including the existing contents. Unfortunately, I had been overzealous
when I wrote a51288aabb, since I gave it
the length of the destination buffer, minus the existing contents. This
likely caused a regression on large strings.

On the topic of being overzealous, the use of strlcat() in
dmu_send_estimate_fast() was unnecessary because recv_clone_name is a
fixed length string. We continue using strlcat() mostly as defensive
programming, in case the string length is ever changed, even though it
is unnecessary.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14476

lib/libzfs/libzfs_sendrecv.c

@@ -4590,7 +4590,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 			    B_FALSE, destsnap) == 0) {
 				*strchr(destsnap, '@') = '\0';
 				(void) strlcat(destsnap, suffix,
-				    sizeof (destsnap) - strlen(destsnap));
+				    sizeof (destsnap));
 			}
 		}
 	} else {
@@ -4626,7 +4626,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 			    B_FALSE, destsnap) == 0) {
 				*strchr(destsnap, '@') = '\0';
 				(void) strlcat(destsnap, snap,
-				    sizeof (destsnap) - strlen(destsnap));
+				    sizeof (destsnap));
 			}
 		}
 	}