xfs: restore speculative_cow_prealloc_lifetime sysctl

mainline inclusion
from mainline-v5.11-rc4
commit 89e0eb8c
category: bugfix
bugzilla: 187526,https://gitee.com/openeuler/kernel/issues/I4KIAO

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=89e0eb8c13bb842e224b27d7e071262cd84717cb



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In commit 9669f51d I tried to get rid of the undocumented cow gc
lifetime knob.  The knob's function was never documented and it now
doesn't really have a function since eof and cow gc have been
consolidated.

Regrettably, xfs/231 relies on it and regresses on for-next.  I did not
succeed at getting far enough through fstests patch review for the fixup
to land in time.

Restore the sysctl knob, document what it did (does?), put it on the
deprecation schedule, and rip out a redundant function.

Fixes: 9669f51d ("xfs: consolidate the eofblocks and cowblocks workers")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Guo Xuenan <guoxuenan@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>