MFC r272044:

Constify a parameter of name2oid.  No functional change.
MFC r274303:

Apply upstream 13597:3eac1e8e0f4c (git: illumos-gate@aa846ad9):

Initialize tqent_flags in the userland taskq implementation.  Without
this the assertion of tq->tq_freelist != NULL may fail in taskq_destroy.

The problem is that tqent_flags is never initialized in the userland
implementation while the kernel one does initialize it.  Without proper
initialization, the flag may have its lowest bit set, making it treated
as TQENT_FLAG_PREALLOC and never removing taskq_ent_t from tq_freelist.
MFC r274458,274523:

Derive copyright year from src/COPYRIGHT.
MFC r274276: MFV r274271:

Improve zdb -b performance:

 - Reduce gethrtime() call to 1/100th of blkptr's;
 - Skip manipulating the size-ordered tree;
 - Issue more (10, previously 3) async reads;
 - Use lighter weight testing in traverse_visitbp();

Illumos issue:
    5243 zdb -b could be much faster
MFC r274636:

Sync with userland variant.
MFC r274674:

Add a tunable for spa_slop_shift which controls how much space we
would reserve by default.  Tuning is not recommended.

Relnotes:	yes
MFC r274370:

Rename variable name from 'index' to 'idx' to avoid shadowing index(3).

Noticed by:	dim
MFC r274172 (avg)

fix l2arc compression buffers leak

We have observed that arc_release() can be called concurrently with a
l2arc in-flight write.
Also, we have observed that arc_hdr_destroy() can be called from
arc_write_done() for a zio with ZIO_FLAG_IO_REWRITE flag in similar
circumstances.

Previously the l2arc headers would be freed while leaking their
associated compression buffers.  Now the buffers are placed on
l2arc_free_on_write list for delayed freeing.  This is similar to what
was already done to arc buffers that were supposed to be freed
concurrently with in-flight writes of those buffers.

In addition to fixing the discovered leaks this change also adds some
protective code to assert that a compression buffer associated with a
l2arc header is never leaked.

A new kstat l2_cdata_free_on_write is added.  It keeps a count of
delayed compression buffer frees which previously would have been leaks.

Tested by:      Vitalij Satanivskij <satan@ukr.net> et al
Requested by:   many
Sponsored by:   HybridCluster / ClusterHQ

This is a 10.1-RELEASE errata candidate.