Fix problem in last change to the procfs: Forgot to save the reallocated table pointer!

Also added warnings:  There are some concurrency issues in the current implementation if you try to modify the procfs data structures will the procfs is mounted and possibly busy.

include/nuttx/fs/procfs.h

@@ -186,6 +186,10 @@ size_t procfs_memcpy(FAR const char *src, size_t srclen,
  * Description:
  *   Add a new entry to the procfs file system.
  *
+ *   NOTE: This function should be called *prior* to mounting the procfs
+ *   file system to prevent concurrency problems with the modification of
+ *   the procfs data set while it is in use.
+ *
  * Input Parameters:
  *   entry - Describes the entry to be registered.
  *