virtiofsd: Let meson check for statx.stx_mnt_id

In virtiofsd, we assume that the presence of the STATX_MNT_ID macro
implies existence of the statx.stx_mnt_id field.  Unfortunately, that is
not necessarily the case: glibc has introduced the macro in its commit
88a2cf6c4bab6e94a65e9c0db8813709372e9180, but the statx.stx_mnt_id field
is still missing from its own headers.

Let meson.build actually chek for both STATX_MNT_ID and
statx.stx_mnt_id, and set CONFIG_STATX_MNT_ID if both are present.
Then, use this config macro in virtiofsd.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/882
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220223092340.9043-1-hreitz@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

meson.build

@@ -1306,6 +1306,18 @@ statx_test = gnu_source_prefix + '''
 
 has_statx = cc.links(statx_test)
 
+# Check whether statx() provides mount ID information
+
+statx_mnt_id_test = gnu_source_prefix + '''
+  #include <sys/stat.h>
+  int main(void) {
+    struct statx statxbuf;
+    statx(0, "", 0, STATX_BASIC_STATS | STATX_MNT_ID, &statxbuf);
+    return statxbuf.stx_mnt_id;
+  }'''
+
+has_statx_mnt_id = cc.links(statx_mnt_id_test)
+
 have_vhost_user_blk_server = get_option('vhost_user_blk_server') \
   .require(targetos == 'linux',
            error_message: 'vhost_user_blk_server requires linux') \
@@ -1553,6 +1565,7 @@ config_host_data.set('CONFIG_NETTLE', nettle.found())
 config_host_data.set('CONFIG_QEMU_PRIVATE_XTS', xts == 'private')
 config_host_data.set('CONFIG_MALLOC_TRIM', has_malloc_trim)
 config_host_data.set('CONFIG_STATX', has_statx)
+config_host_data.set('CONFIG_STATX_MNT_ID', has_statx_mnt_id)
 config_host_data.set('CONFIG_ZSTD', zstd.found())
 config_host_data.set('CONFIG_FUSE', fuse.found())
 config_host_data.set('CONFIG_FUSE_LSEEK', fuse_lseek.found())

tools/virtiofsd/passthrough_ll.c

@@ -1039,7 +1039,7 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
 {
     int res;
 
-#if defined(CONFIG_STATX) && defined(STATX_MNT_ID)
+#if defined(CONFIG_STATX) && defined(CONFIG_STATX_MNT_ID)
     if (lo->use_statx) {
         struct statx statxbuf;