mdmfs: Fix many bugs in automatic md(4) creation.

This code allocated a correctly sized buffer, read past the end of the
source buffer, writing off the end of the target buffer, and then writing
a '\0' terminator past the end of the target buffer (in the wrong place).
It then leaked the buffer.

Switch to a statically sized buffer on the stack and update the source
pointer and
length before use so the correct things are copied.

Fix a logic error in the checks that the format of the line is as
expected and move on out of an assert.

Remove an unneeded close(). fclose() closes the descriptor.

Found with: CheriABI
Obtained from: CheriBSD
Reviewed by: kib, jhb, markj
Differential Revision: https://reviews.freebsd.org/D19122