md: don't leave 'MD_RECOVERY_FROZEN' in error path of md_set_readonly()

If md_set_readonly() failed, the array could still be read-write, however
'MD_RECOVERY_FROZEN' could still be set, which leave the array in an
abnormal state that sync or recovery can't continue anymore.
Hence make sure the flag is cleared after md_set_readonly() returns.

Fixes: 88724bfa ("md: wait for pending superblock updates before switching to read-only")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231205094215.1824240-3-yukuai1@huaweicloud.com