ssh_filter_btrbk: update ssh_filter_btrbk.sh to use readlink

bugfix for: 796b6bd9bf

Replace realpath with readlink in allowed commands. Commit 796b6bd substituted readlink for realpath in file "btrbk"; this commit propagates the change to ssh_filter_btrbk.sh.

ChangeLog

@@ -1,3 +1,7 @@
+btrbk-current
+
+  * Bugfix: Replace "realpath" with "readlink" in ssh_filter_btrbk.sh
+
 btrbk-0.23.3
 
   * Replace "realpath -e" with "readlink -e" for mountpoint discovery

ssh_filter_btrbk.sh

@@ -102,7 +102,7 @@ while [[ "$#" -ge 1 ]]; do
       -t|--target)
           allow_cmd "btrfs receive"
           # the following are needed if targets point to a directory
-          allow_cmd "realpath"
+          allow_cmd "readlink"
           allow_exact_cmd "cat /proc/self/mounts"
           ;;