vfs: remove lockdep fs freeze weirdness

A long time ago, XFS got tangled up with lockdep because the last thing
it does during a fs freeze is use a transaction to flush the superblock
to disk.  Transactions require int(ernal) write freeze protection, which
implies a recursive grab of the intwrite lock and makes lockdep all sad.

This was "solved" in lockdep back in 2012 as commit 5accdf82ba25c by
adding a "convert XFS' blocking fsfreeze lock attempts into a trylock"
behavior in v6 of a patch[1] that Jan Kara sent to try to fix fs freeze
handling.  The behavior was not in the v5[0] patch, nor was there any
discussion for any of the v5 patches that would suggest why things
changed from v5 to v6.

Commit f4b554af9931 in 2015 created the current weird logic in
__sb_start_write, which converts recursive freeze lock grabs into
trylocks whose return values are ignored(!!!).  XFS solved the problem
by creating a variant of transactions (XFS_TRANS_NO_WRITECOUNT) that
don't grab intwrite freeze protection, thus making lockdep's solution
unnecessary.  The commit claims that Dave Chinner explained that the
trylock hack + comment could be removed, but the patch author never did
that, and lore doesn't seem to know where or when Dave actually said
that?

Now it's 2020, and still nobody removed this from __sb_start_write.
Worse yet, nowadays lock_is_held returns 1 if lockdep is built-in but
offline.  This causes attempts to grab the pagefaults freeze lock
synchronously to turn into unchecked trylocks!  Hilarity ensues if a
page fault races with fsfreeze and loses, which causes us to break the
locking model.

This finally came to a head in 5.10-rc1 because the new lockdep bugs
introduced during the merge window caused this maintainer to hit the
weird case where sb_start_pagefault can return without having taken the
freeze lock, leading to test failures and memory corruption.

Since this insanity is dangerous and hasn't been needed by xfs since the
late 2.6(???) days, kill it with fire.

v2: refactor helpers to be more cohesive
v3: move more cohesive helpers to header files