cpukit/libfs: Correct comparisons of different signedness

The GCC warning -Wsign-compare flagged these instances of comparing
signed and unsigned types. A common pair was size_t and int.

cpukit/libfs/src/imfs/imfs_symlink.c

@@ -80,7 +80,7 @@ ssize_t IMFS_readlink(
 )
 {
   IMFS_sym_link_t   *sym_link;
-  ssize_t            i;
+  size_t             i;
 
   sym_link = loc->node_access;
 

cpukit/libfs/src/pipe/fifo.c

@@ -353,7 +353,10 @@ ssize_t pipe_write(
   rtems_libio_t  *iop
 )
 {
-  int chunk, chunk1, written = 0, ret = 0;
+  int ret = 0;
+  size_t chunk1;
+  size_t written = 0;
+  size_t chunk;
 
   if (!pipe)
     return -EPIPE;

cpukit/libfs/src/rfs/rtems-rfs-trace.c

@@ -117,7 +117,7 @@ rtems_rfs_trace_shell_command (int argc, char *argv[])
   rtems_rfs_trace_mask clear_value = 0;
   bool                 set = true;
   int                  arg;
-  int                  t;
+  size_t               t;
 
   for (arg = 1; arg < argc; arg++)
   {