btrfs-util: add helper that abstracts "might be btrfs subvol?" check

Let#s not hardcode inode nr 256 everywhere, but abstract this check
slightly.

src/basic/btrfs-util.c

@@ -91,7 +91,7 @@ int btrfs_is_subvol_fd(int fd) {
         if (fstat(fd, &st) < 0)
                 return -errno;
 
-        if (!S_ISDIR(st.st_mode) || st.st_ino != 256)
+        if (!btrfs_might_be_subvol(&st))
                 return 0;
 
         return btrfs_is_filesystem(fd);
@@ -194,7 +194,7 @@ int btrfs_subvol_set_read_only_fd(int fd, bool b) {
         if (fstat(fd, &st) < 0)
                 return -errno;
 
-        if (!S_ISDIR(st.st_mode) || st.st_ino != 256)
+        if (!btrfs_might_be_subvol(&st))
                 return -EINVAL;
 
         if (ioctl(fd, BTRFS_IOC_SUBVOL_GETFLAGS, &flags) < 0)
@@ -229,7 +229,7 @@ int btrfs_subvol_get_read_only_fd(int fd) {
         if (fstat(fd, &st) < 0)
                 return -errno;
 
-        if (!S_ISDIR(st.st_mode) || st.st_ino != 256)
+        if (!btrfs_might_be_subvol(&st))
                 return -EINVAL;
 
         if (ioctl(fd, BTRFS_IOC_SUBVOL_GETFLAGS, &flags) < 0)

src/basic/btrfs-util.h

@@ -127,3 +127,13 @@ static inline int btrfs_log_dev_root(int level, int ret, const char *p) {
                               "File system behind %s is reported by btrfs to be backed by pseudo-device /dev/root, which is not a valid userspace accessible device node. "
                               "Cannot determine correct backing block device.", p);
 }
+
+static inline bool btrfs_might_be_subvol(const struct stat *st) {
+        if (!st)
+                return false;
+
+        /* Returns true if this 'struct stat' looks like it could refer to a btrfs subvolume. To make a final
+         * decision, needs to be combined with an fstatfs() check to see if this is actually btrfs. */
+
+        return S_ISDIR(st->st_mode) && st->st_ino == 256;
+}

src/basic/rm-rf.c

@@ -147,7 +147,7 @@ int rm_rf_children(int fd, RemoveFlags flags, struct stat *root_dev) {
                         if (r > 0)
                                 continue;
 
-                        if ((flags & REMOVE_SUBVOLUME) && st.st_ino == 256) {
+                        if ((flags & REMOVE_SUBVOLUME) && btrfs_might_be_subvol(&st)) {
 
                                 /* This could be a subvolume, try to remove it */