Allow mysqld_t to read and write to the 'memory.pressure' file in cgroup2

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Note:
The original suggestion was:
  allow mysqld_t cgroup_t:file { read write };
however one should not use a SELinux type from outside of their own SELinux module

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Note from Daniel Black:
For clarity
  MariaDB/server@2323483#diff-ed06407705f2d1088e796ecb0c9592f1928f7b86fa8e48cbbe50f589fce18f3cR801
is the write to describe the PSI event desired from the kernel.
ref: https://www.kernel.org/doc/html/latest/accounting/psi.html

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Resolves: RHBZ#2294899 RHBZ#2256002

Author: FaramosCZ

mysql.te

@@ -79,6 +79,10 @@ allow mysqld_t self:udp_socket create_socket_perms;
 kernel_read_network_state(mysqld_t)
 kernel_read_net_sysctls(mysqld_t)
 
+# Allow mysqld_t to read to memory.pressure in cgroup
+fs_read_cgroup_files(mysqld_t)
+fs_write_cgroup_files(mysqld_t)
+
 manage_dirs_pattern(mysqld_t, mysqld_db_t, mysqld_db_t)
 manage_files_pattern(mysqld_t, mysqld_db_t, mysqld_db_t)
 manage_sock_files_pattern(mysqld_t, mysqld_db_t, mysqld_db_t)