stat-util: expose fd_is_read_only_fs()

Currently it is not used, but the fucntion is already quite generic and
useful.

src/basic/stat-util.c

@@ -239,7 +239,7 @@ int null_or_empty_path_with_root(const char *fn, const char *root) {
         return null_or_empty(&st);
 }
 
-static int fd_is_read_only_fs(int fd) {
+int fd_is_read_only_fs(int fd) {
         struct statvfs st;
 
         assert(fd >= 0);

src/basic/stat-util.h

@@ -45,6 +45,7 @@ static inline int null_or_empty_path(const char *fn) {
         return null_or_empty_path_with_root(fn, NULL);
 }
 
+int fd_is_read_only_fs(int fd);
 int path_is_read_only_fs(const char *path);
 
 int inode_same_at(int fda, const char *filea, int fdb, const char *fileb, int flags);