src/basic/missing_loop.h: fix missing LOOP_SET_BLOCK_SIZE

Builds with kernels headers < 4.14 fail with:

../src/shared/loop-util.c: In function ‘loop_configure_fallback’:
../src/shared/loop-util.c:237:31: error: ‘LOOP_SET_BLOCK_SIZE’ undeclared (first use in this function); did you mean ‘LOOP_SET_DIRECT_IO’?
                 if (ioctl(fd, LOOP_SET_BLOCK_SIZE, (unsigned long) c->block_size) < 0)
                               ^~~~~~~~~~~~~~~~~~~
                               LOOP_SET_DIRECT_IO

Fixes: systemd/systemd#33341

Signed-off-by: Raphaël Mélotte <[email protected]>
(cherry picked from commit 56ab1c5)
(cherry picked from commit 0730ec4)
(cherry picked from commit ed087e8)

Author: rmelotte

src/basic/missing_loop.h

@@ -22,3 +22,7 @@ struct loop_config {
 #ifndef LOOP_SET_STATUS_SETTABLE_FLAGS
 #define LOOP_SET_STATUS_SETTABLE_FLAGS (LO_FLAGS_AUTOCLEAR | LO_FLAGS_PARTSCAN)
 #endif
+
+#ifndef LOOP_SET_BLOCK_SIZE
+#  define LOOP_SET_BLOCK_SIZE 0x4C09
+#endif