MFC r271218:
Update mixer description for FastTrackPro.

Approved by:	re, marius
MFC r271222:

Fix typo.

Submitted by:	Dmitry Morozovsky <marck rinet ru>
Approved by:	re (gjb)
MFC r271226: MFV r271223:

In dnode_sync(), do dnode_increase_indirection() before processing
the dn_next_nblkptr.

Illumos issue:
    5117 space map reallocation can cause corruption

Approved by:	re (gjb)
MFC 270823,270825,270829:
Use a unit number allocator to provide suitable st_dev and st_ino values
for POSIX shared memory descriptors.  The implementation is similar to
that used for pipes.

Approved by:	re (gjb for 10)