MFV r275783:

Convert ARC flags to use enum.  Previously, public flags are defined in
arc.h and private flags are defined in arc.c which can lead to confusion
and programming errors.

Consistently use 'hdr' (when referencing arc_buf_hdr_t) instead of 'buf'
or 'ab' because arc_buf_t are often named 'buf' as well.

Illumos issue:
    5369 arc flags should be an enum
    5370 consistent arc_buf_hdr_t naming scheme

MFC after:	2 weeks
MFV r275784:

Plug a memory leak in libzfs.  In zfs_iter_bookmarks, an nvlist is allocated
before calling lzc_get_bookmarks, which allocates the nvlist again (and
overwrites the pointer to previously allocated list).

Illumos issue:
    5427 memory leak in libzfs when doing rollback

MFC after:	2 weeks