gfapi: dump the active inode table in statedumps

[ThalesBarretto]

gfapi: dump the active inode table in statedumps

Problem

A gfapi process's statedump has never contained its inode table: the table hangs off the active graph's top
xlator (graph_setup), which has no dumpops, and the generic per-xlator itable dump in statedump.c has been
dead since 4e40d9d6d7 (2012) — gf_proc_dump_single_xlator_info() still builds the section key and then
never calls inode_table_dump(). FUSE mounts and bricks dump their tables themselves.

Fix

1. Give the gfapi master xlator an .inode dumper modelled on fuse_itable_dump() (gNFS had the same gap closed with
   its own dumper: BZ#1419870, reported 2017, landed 2019 in 8c66ebe35a): section xlator.mount.api.itable with the table sizes and the
   active/lru/purge inodes, each with its refs, per-xlator ref counts (ref_by_xl:), fd counts, and the
   per-xlator inode and fd contexts where an xlator set one.
2. Remove the dead itable_key in gf_proc_dump_single_xlator_info() so nobody reads it as a working generic
   itable dump.

What it shows

A gfapi program that opens 10 files and closes them without I/O under performance.open-behind on (the leak of
#3977), statedump via glfs_sysrq(), on a stock devel build with this change:

[xlator.mount.api.itable]
xlator.mount.api.itable.active_size=11
xlator.mount.api.itable.lru_size=0
[xlator.mount.api.itable.active.1]
gfid=37a46317-6c31-4f6f-a0bd-69de3ad56230
fd-count=1
active-fd-count=1
ref=2
ref_by_xl:.gfapi=1
ref_by_xl:.vol4490-open-behind=1

and with the #3977 fix applied: active_size=1, lru_size=10, the file inodes idle with fd-count=0 ref=0.
Without this change the same dump has no inode information at all (0 itable sections).

Notes for review: inode_table_dump() takes the table lock with trylock and skips the table when busy, as it
does for FUSE; fs->active_subvol is read without the fs lock, as fuse_itable_dump() reads
priv->active_subvol; only the active graph's table is dumped (old graphs still carry the 2012 TODO in
gf_proc_dump_oldgraph_xlator_info()); the section obeys the inode statedump option, so a consumer with a
very large table can turn it off through glusterdump.options.

Fixes: #4838