makefs: clean up warnings

zfs/fs.c:
zfs/objset.c:
zfs/vdev.c:
zfs/zap.c:
Add include sys/param.h

dsl_dir_alloc() needs to set parent = NULL to silence warning
about 'parent' may be used uninitialized. Warning is given because
we break the loop when nextdir == NULL and parent was not previously set.
(it should not happen, but compiler does not know that).

zap_add() and zap_fat_write_array_chunk() takes uint8_t *, use type
cast.

zap_fat_write_array_chunk() should check sz for 0 to avoid
use of uninitialized pointer.

unchecked function returns.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D51592