tree-wide: fix spelling of "fallback"

Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back"
is the verb. (This is pretty clear when we construct a sentence in the
present continous: "we are falling back" not "we are fallbacking").

src/shared/dev-setup.c

@@ -72,10 +72,11 @@ int make_inaccessible_nodes(
                 { "inaccessible/sock", S_IFSOCK | 0000 },
 
                 /* The following two are likely to fail if we lack the privs for it (for example in an userns
-                 * environment, if CAP_SYS_MKNOD is missing, or if a device node policy prohibit major/minor of 0
-                 * device nodes to be created). But that's entirely fine. Consumers of these files should carry
-                 * fallback to use a different node then, for example <root>/inaccessible/sock, which is close
-                 * enough in behaviour and semantics for most uses. */
+                 * environment, if CAP_SYS_MKNOD is missing, or if a device node policy prohibits creation of
+                 * device nodes with a major/minor of 0). But that's entirely fine. Consumers of these files
+                 * should implement falling back to use a different node then, for example
+                 * <root>/inaccessible/sock, which is close enough in behaviour and semantics for most uses.
+                 */
                 { "inaccessible/chr",  S_IFCHR  | 0000 },
                 { "inaccessible/blk",  S_IFBLK  | 0000 },
         };