MFV r268455:

Use reserved space for ZFS administrative commands.

We reserve 1/2^spa_slop_shift = 1/32 or 3.125% of pool space (or 32MB at
least) for system use.  Most ZPL operations, e.g. write(2), creat(2), will
fail with ENOSPC if we fall below this.

Certain operations, e.g. file removal and most administrative actions,
still permitted until half of the slop space is used.  This would allow
users to use these operations to free up space in the pool when pool is
close to full but half of slop space is still free.

A very restricted set of operations that frees up space or change quota
are always permitted, regardless of the amount of free space.

MFC after:	 2 weeks