Turn 'rm_entries' in 'struct ocfs2_recovery_map' into a flexible array.

The advantages are:
   - save the size of a pointer when the new undo structure is allocated
   - avoid some always ugly pointer arithmetic to get the address of
    'rm_entries'
   - avoid an indirection when the array is accessed

While at it, use struct_size() to compute the size of the new undo
structure.

Link: https://lkml.kernel.org/r/c645911ffd2720fce5e344c17de642518cd0db52.1689533270.git.christophe.jaillet@wanadoo.frSigned-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>