fs/locks.c: prepare for BKL removal

This prepares the removal of the big kernel lock from the
file locking code. We still use the BKL as long as fs/lockd
uses it and ceph might sleep, but we can flip the definition
to a private spinlock as soon as that's done.
All users outside of fs/lockd get converted to use
lock_flocks() instead of lock_kernel() where appropriate.

Based on an earlier patch to use a spinlock from Matthew
Wilcox, who has attempted this a few times before, the
earliest patch from over 10 years ago turned it into
a semaphore, which ended up being slower than the BKL
and was subsequently reverted.

Someone should do some serious performance testing when
this becomes a spinlock, since this has caused problems
before. Using a spinlock should be at least as good
as the BKL in theory, but who knows...

Signed-off-by: Arnd Bergmann <[email protected]>
Acked-by: Matthew Wilcox <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: "J. Bruce Fields" <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: John Kacur <[email protected]>
Cc: Sage Weil <[email protected]>
Cc: [email protected]
Cc: [email protected]

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