hfs: replace kmap() with kmap_local_page() in btree.c

kmap() is being deprecated in favor of kmap_local_page().

Two main problems with kmap(): (1) It comes with an overhead as mapping
space is restricted and protected by a global lock for synchronization and
(2) it also requires global TLB invalidation when the kmap's pool wraps
and it might block when the mapping space is fully utilized until a slot
becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts). 
It is faster than kmap() in kernels with HIGHMEM enabled.  Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and still valid.

Since its use in btree.c is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in btree.c.  Where
possible, use the suited standard helpers (memzero_page(), memcpy_page())
instead of open coding kmap_local_page() plus memset() or memcpy().

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Fabio M. De Francesco <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Reviewed-by: Viacheslav Dubeyko <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Chaitanya Kulkarni <[email protected]>
Cc: Christian Brauner (Microsoft) <[email protected]>
Cc: Damien Le Moal <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Martin K. Petersen <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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