block/export/fuse.c: fix musl build

Fix the following build failure on musl raised since version 6.0.0 and
https://gitlab.com/qemu-project/qemu/-/commit/4ca37a96a75aafe7a37ba51ab1912b09b7190a6b
because musl does not define FALLOC_FL_ZERO_RANGE:

../block/export/fuse.c: In function 'fuse_fallocate':
../block/export/fuse.c:563:23: error: 'FALLOC_FL_ZERO_RANGE' undeclared (first use in this function)
  563 |     } else if (mode & FALLOC_FL_ZERO_RANGE) {
      |                       ^~~~~~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/b96e3d364fd1f8bbfb18904a742e73327d308f64

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Message-Id: <20210809095101.1101336-1-fontaine.fabrice@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

Author: ffontaine

block/export/fuse.c

@@ -635,7 +635,9 @@ static void fuse_fallocate(fuse_req_t req, fuse_ino_t inode, int mode,
             offset += size;
             length -= size;
         } while (ret == 0 && length > 0);
-    } else if (mode & FALLOC_FL_ZERO_RANGE) {
+    }
+#ifdef CONFIG_FALLOCATE_ZERO_RANGE
+    else if (mode & FALLOC_FL_ZERO_RANGE) {
         if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + length > blk_len) {
             /* No need for zeroes, we are going to write them ourselves */
             ret = fuse_do_truncate(exp, offset + length, false,
@@ -654,7 +656,9 @@ static void fuse_fallocate(fuse_req_t req, fuse_ino_t inode, int mode,
             offset += size;
             length -= size;
         } while (ret == 0 && length > 0);
-    } else if (!mode) {
+    }
+#endif /* CONFIG_FALLOCATE_ZERO_RANGE */
+    else if (!mode) {
         /* We can only fallocate at the EOF with a truncate */
         if (offset < blk_len) {
             fuse_reply_err(req, EOPNOTSUPP);