fs-util: increase start buffer size in readlinkat_malloc()

I noticed while profiling journald that we invoke readlinkat() a ton on
open /proc/self/fd/<fd>, and that the returned paths are more often than
not longer than the 99 chars used before, when we look at archived
journal files. This means for these cases we generally need to execute
two rather than one syscalls.

Let's increase the buffer size a tiny bit, so that we reduce the number
of syscalls executed. This is really a low-hanging fruit of
optimization.

src/basic/fs-util.c

@@ -132,7 +132,7 @@ int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char
 }
 
 int readlinkat_malloc(int fd, const char *p, char **ret) {
-        size_t l = 100;
+        size_t l = FILENAME_MAX+1;
         int r;
 
         assert(p);