Ensure fragment_vector fragments are always <= 128KiB

[ballard26]

Previously fragment_vector<T>::pow2_floor(size_t size) wouldn't correctly calculate the nearest power of 2 <= size. This resulted in cases such as;

sizeof(cluster::topic_status) * chunked_vector<cluster::topic_status>::elems_per_frag <= 128UL * 1024

not being true. This PR replaces pow2_floor with a standard library equivalent and adds a static assertion to ensure the size of fragments is always <= 128KiB.

Fixes #16957

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x

Release Notes

• none

[piyushredpanda]

Thanks for spotting the bug and the fix, @ballard26 !

[dotnwat]

lgtm

[vbotbuildovich]

/backport v23.3.x