usr.sbin/makefs/hammer2: Introduce XOP fifo realloc

usr.sbin/makefs/hammer2: Introduce XOP fifo realloc

Since makefs HAMMER2 used a serialized single threaded xops,
it was not possible to have more than HAMMER2_XOPFIFO fifo entries.

Change this to realloc XOP fifo whenever it hits current limit.
This is also what Free|Net|OpenBSD HAMMER2 currently do until they
support multi-threaded xops.

This makes possible to support VOP_READDIR, and that makes possible
to support recursive read for "-o R" option.