xfs: factor out xfs_destroy_perag()

Offering: HULK
hulk inclusion
category: bugfix
bugzilla: 188878, https://gitee.com/openeuler/kernel/issues/I76JSK



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Factor out xfs_destroy_perag() from xfs_initialize_perag() for error
handle, delete perag from radix tree requires lock protection, just like
any other places where perag tree are modified.

Signed-off-by: Long Li <leo.lilong@huawei.com>