MDEV-14499 Mariabackup 10.2 fails to back up a multi-file InnoDB system tablespace

When Mariabackup is invoked on an instance that uses a multi-file
InnoDB system tablespace, it may fail to other files of the system
tablespace than the first one.
This was revealed by the MDEV-14447 test case.
The offending code is assuming that the first page of each data file
is page 0. But, in multi-file system tablespaces that is not the case.

xb_fil_cur_open(): Instead of re-reading the first page of the file,
rely on the fil_space_t metadata that already exists in memory.

xb_get_space_flags(): Remove.

extra/mariabackup/fil_cur.cc

@@ -212,24 +212,7 @@ xb_fil_cur_open(
 
 	posix_fadvise(cursor->file, 0, 0, POSIX_FADV_SEQUENTIAL);
 
-	/* Determine the page size */
-	ulint	flags = xb_get_space_flags(cursor->file);
-	if (flags == ULINT_UNDEFINED) {
-		xb_fil_cur_close(cursor);
-		return(XB_FIL_CUR_SKIP);
-	}
-
-	if (!fsp_flags_is_valid(flags, cursor->space_id)) {
-		ulint cflags = fsp_flags_convert_from_101(flags);
-		if (cflags == ULINT_UNDEFINED) {
-			msg("[%02u] mariabackup: Error: Invalid "
-			    "tablespace flags: %x.\n", thread_n, uint(flags));
-			return(XB_FIL_CUR_SKIP);
-		}
-		flags = cflags;
-	}
-
-	const page_size_t page_size(flags);
+	const page_size_t page_size(cursor->node->space->flags);
 	cursor->page_size = page_size;
 
 	/* Allocate read buffer */

extra/mariabackup/xtrabackup.cc

@@ -2137,28 +2137,6 @@ check_if_skip_table(
 	return(FALSE);
 }
 
-/** @return the tablespace flags from a given data file
-@retval	ULINT_UNDEFINED	if the file is not readable */
-ulint xb_get_space_flags(pfs_os_file_t file)
-{
-	byte	*buf;
-	byte	*page;
-	ulint	flags;
-
-	buf = static_cast<byte *>(malloc(2 * UNIV_PAGE_SIZE));
-	page = static_cast<byte *>(ut_align(buf, UNIV_PAGE_SIZE));
-
-	if (os_file_read(IORequestRead, file, page, 0, UNIV_PAGE_SIZE)) {
-		flags = fsp_header_get_flags(page);
-	} else {
-		flags = ULINT_UNDEFINED;
-	}
-
-	free(buf);
-
-	return(flags);
-}
-
 const char*
 xb_get_copy_action(const char *dflt)
 {

extra/mariabackup/xtrabackup.h

@@ -149,10 +149,6 @@ void xtrabackup_io_throttling(void);
 my_bool xb_write_delta_metadata(const char *filename,
 				const xb_delta_info_t *info);
 
-/** @return the tablespace flags from a given data file
-@retval	ULINT_UNDEFINED	if the file is not readable */
-ulint xb_get_space_flags(pfs_os_file_t file);
-
 /************************************************************************
 Checks if a table specified as a name in the form "database/name" (InnoDB 5.6)
 or "./database/name.ibd" (InnoDB 5.5-) should be skipped from backup based on