Fixed metadata_max==prog_size commit->end calculation
The inconsistency here between the use of block_size vs metadata_max was
suspicious. Turns out there's a bug when metadata_max == prog_size.
We correctly use metadata_max for the block_size/2 check, but we weren't
using it for the block_size-40 check. The second check seems unnecessary
after the first, but it protects against running out of space in a
commit for commit-related metadata (checksums, tail pointers, etc) when
we can't program half-blocks.
Turns out this is also needed when limiting metadata_max to a single
prog, otherwise we risk erroring with LFS_ERR_NOSPC early.
Found by ajheck, dpkristensen, NLLK, and likely others.
Christopher Haster