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/basic/fd-util.c

@@ -976,6 +976,6 @@ int read_nr_open(void) {
                         return v;
         }
 
-        /* If we fail, fallback to the hard-coded kernel limit of 1024 * 1024. */
+        /* If we fail, fall back to the hard-coded kernel limit of 1024 * 1024. */
         return 1024 * 1024;
 }

src/basic/fs-util.c

@@ -120,7 +120,7 @@ int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char
         if (!IN_SET(errno, EINVAL, ENOSYS, ENOTTY, EPERM)) /* FAT returns EPERM on link()… */
                 return -errno;
 
-        /* OK, neither RENAME_NOREPLACE nor linkat()+unlinkat() worked. Let's then fallback to the racy TOCTOU
+        /* OK, neither RENAME_NOREPLACE nor linkat()+unlinkat() worked. Let's then fall back to the racy TOCTOU
          * vulnerable accessat(F_OK) check followed by classic, replacing renameat(), we have nothing better. */
 
         if (faccessat(newdirfd, newpath, F_OK, AT_SYMLINK_NOFOLLOW) >= 0)

src/basic/mountpoint-util.c

@@ -173,7 +173,7 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
                 if (!ERRNO_IS_NOT_SUPPORTED(errno) && !ERRNO_IS_PRIVILEGE(errno))
                         return -errno;
 
-                /* If statx() is not available or forbidden, fallback to name_to_handle_at() below */
+                /* If statx() is not available or forbidden, fall back to name_to_handle_at() below */
         } else if (FLAGS_SET(sx.stx_attributes_mask, STATX_ATTR_MOUNT_ROOT)) /* yay! */
                 return FLAGS_SET(sx.stx_attributes, STATX_ATTR_MOUNT_ROOT);
 
@@ -186,7 +186,7 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
                 goto fallback_fdinfo;
         else if (r == -EOPNOTSUPP)
                 /* This kernel or file system does not support name_to_handle_at(), hence let's see if the upper fs
-                 * supports it (in which case it is a mount point), otherwise fallback to the traditional stat()
+                 * supports it (in which case it is a mount point), otherwise fall back to the traditional stat()
                  * logic */
                 nosupp = true;
         else if (r < 0)