osd: speed up upmap underfull fallback

According to our tests, this commit can speed up pg upmap calculations
by orders of magnitude

Performance analysis:
Suppose:
R = number of pg_upmap_items (the full candidate pool, after to_skip/only_pools filtering)
M = how many OSDs the deviation_osd loop visits before it breaks (less or equal to underfull count)
U = the number of distinct from OSDs in the upmap items. U is less or equal to number of OSDs, and should be less than R x k
k = mapping pairs per candidate (usually 1, small)

Time complexity of OSDMap::calc_pg_upmaps():
Before the commit: O(M * R)
After the commit: O(R * k + M * log(U)) ~= O(R + M *log(U))

Performant evaluation:
Existing upmap items    Underfull OSDs  Old path        New path        Speedup
50000                   1000            6.51s           0.047s          137x
100000                  1000            12.05s          0.086s          140x
200000                  2000            63.93s          0.281s          227x

Fixes: https://tracker.ceph.com/issues/77563
Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>
Assisted-by: Codex:GPT-5.5