fd-util: rename loop_get_diskseq() -> fd_get_diskseq()

And move it from loop-util.[ch] -> fd-util.[ch]

src/basic/fd-util.c

@@ -3,6 +3,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/btrfs.h>
+#include <linux/fs.h>
 #include <linux/magic.h>
 #include <sys/ioctl.h>
 #include <sys/resource.h>
@@ -17,6 +18,7 @@
 #include "io-util.h"
 #include "macro.h"
 #include "missing_fcntl.h"
+#include "missing_fs.h"
 #include "missing_syscall.h"
 #include "parse-util.h"
 #include "path-util.h"
@@ -788,3 +790,24 @@ int btrfs_defrag_fd(int fd) {
 
         return RET_NERRNO(ioctl(fd, BTRFS_IOC_DEFRAG, NULL));
 }
+
+int fd_get_diskseq(int fd, uint64_t *ret) {
+        uint64_t diskseq;
+
+        assert(fd >= 0);
+        assert(ret);
+
+        if (ioctl(fd, BLKGETDISKSEQ, &diskseq) < 0) {
+                /* Note that the kernel is weird: non-existing ioctls currently return EINVAL
+                 * rather than ENOTTY on loopback block devices. They should fix that in the kernel,
+                 * but in the meantime we accept both here. */
+                if (!ERRNO_IS_NOT_SUPPORTED(errno) && errno != EINVAL)
+                        return -errno;
+
+                return -EOPNOTSUPP;
+        }
+
+        *ret = diskseq;
+
+        return 0;
+}

src/basic/fd-util.h

@@ -109,6 +109,7 @@ static inline int make_null_stdio(void) {
 int fd_reopen(int fd, int flags);
 int read_nr_open(void);
 int btrfs_defrag_fd(int fd);
+int fd_get_diskseq(int fd, uint64_t *ret);
 
 /* The maximum length a buffer for a /proc/self/fd/<fd> path needs */
 #define PROC_FD_PATH_MAX \