[PATCH] fs: remove power of 2 and length boundary atomic write restrictions

[Vitaliy Filippov]

I think that even with the 2^N requirement the user still has to look
for boundaries.
1) NVMe disks may have NABO != 0 (atomic boundary offset). In this
case 2^N aligned writes won't work at all.
2) NABSPF is expressed in blocks in the NVMe spec and it's not
restricted to 2^N, it can be for example 3 (3*4096 = 12 KB). The spec
allows it. 2^N breaks this case too.
And the user also has to look for the maximum atomic write size
anyway, he can't just assume all writes are atomic out of the box,
regardless of the 2^N requirement.
So my idea is that the kernel's task is just to guarantee correctness
of atomic writes. It anyway can't provide the user with atomic writes
in all cases.

I see that xfstests also check 2^N and the check obviously failed with
my patch, should I submit a patch for xfstests first?