MDEV-20286 mariabackup fails when innodb_max_dirty_pages_pct contains a...

MDEV-20286 mariabackup fails when innodb_max_dirty_pages_pct contains a fraction (is not an integer)

This is a port of the Percona Server commit 5265f42e290573e9591f8ca28ab66afc051f89a3

which is the same as their bug PXB-1807: xtrabackup does not accept fractional values for
innodb_max_dirty_pages_pct

Problem:

Variable specified as double in MySQL server, but read as long in the
xtrabackup. This causes xtrabackup to fail at startup when the value
contains decimal point.

Fix:

Make xtrabackup to interpret the value as double to be compatible with
server.

extra/mariabackup/xtrabackup.cc

@@ -1647,8 +1647,11 @@ struct my_option xb_server_options[] =
    "Path to InnoDB log files.", &srv_log_group_home_dir,
    &srv_log_group_home_dir, 0, GET_STR, REQUIRED_ARG, 0, 0, 0, 0, 0, 0},
   {"innodb_max_dirty_pages_pct", OPT_INNODB_MAX_DIRTY_PAGES_PCT,
-   "Percentage of dirty pages allowed in bufferpool.", (G_PTR*) &srv_max_buf_pool_modified_pct,
-   (G_PTR*) &srv_max_buf_pool_modified_pct, 0, GET_ULONG, REQUIRED_ARG, 90, 0, 100, 0, 0, 0},
+   "Percentage of dirty pages allowed in bufferpool.",
+   (G_PTR*) &srv_max_buf_pool_modified_pct,
+   (G_PTR*) &srv_max_buf_pool_modified_pct, 0, GET_DOUBLE, REQUIRED_ARG,
+   (longlong)getopt_double2ulonglong(90), (longlong)getopt_double2ulonglong(0),
+   getopt_double2ulonglong(100), 0, 0, 0},
   {"innodb_use_native_aio", OPT_INNODB_USE_NATIVE_AIO,
    "Use native AIO if supported on this platform.",
    (G_PTR*) &srv_use_native_aio,