stat-util: unify code that checks whether something is a regular file

Let's add a common implementation for regular file checks, that are
careful to return the right error code (EISDIR/EISLNK/EBADFD) when we
are encountering a wrong file node.

src/import/export-raw.c

@@ -37,6 +37,7 @@
 #include "import-common.h"
 #include "missing.h"
 #include "ratelimit.h"
+#include "stat-util.h"
 #include "string-util.h"
 #include "util.h"
 
@@ -319,8 +320,9 @@ int raw_export_start(RawExport *e, const char *path, int fd, ImportCompressType
 
         if (fstat(sfd, &e->st) < 0)
                 return -errno;
-        if (!S_ISREG(e->st.st_mode))
-                return -ENOTTY;
+        r = stat_verify_regular(&e->st);
+        if (r < 0)
+                return r;
 
         /* Try to take a reflink snapshot of the file, if we can t make the export atomic */
         tfd = reflink_snapshot(sfd, path);