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get rid of mountpoint->m_count
struct mountpoint has an odd kinda-sorta refcount in it. It's always
either equal to or one above the number of mounts attached to that
mountpoint.
"One above" happens when a function takes a temporary reference to
mountpoint. Things get simpler if we express that as inserting
a local object into ->m_list and removing it to drop the reference.
New calling conventions:
1) lock_mount(), do_lock_mount(), get_mountpoint() and lookup_mountpoint()
take an extra struct pinned_mountpoint * argument and returns 0/-E...
(or true/false in case of lookup_mountpoint()) instead of returning
struct mountpoint pointers. In case of success, the struct mountpoint *
we used to get can be found as pinned_mountpoint.mp
2) unlock_mount() (always paired with lock_mount()/do_lock_mount()) takes
an address of struct pinned_mountpoint - the same that had been passed to
lock_mount()/do_lock_mount().
3) put_mountpoint() for a temporary reference (paired with get_mountpoint()
or lookup_mountpoint()) is replaced with unpin_mountpoint(), which takes
the address of pinned_mountpoint we passed to matching {get,lookup}_mountpoint().
4) all instances of pinned_mountpoint are local variables; they always live on
stack. {} is used for initializer, after successful {get,lookup}_mountpoint()
we must make sure to call unpin_mountpoint() before leaving the scope and
after successful {do_,}lock_mount() we must make sure to call unlock_mount()
before leaving the scope.
5) all manipulations of ->m_count are gone, along with ->m_count itself.
struct mountpoint lives while its ->m_list is non-empty.
Signed-off-by: 
Al Viro <viro@zeniv.linux.org.uk>