nvmet-tcp: don't map pages which can't come from HIGHMEM

kmap() is being deprecated in favor of kmap_local_page().[1]

There are 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.

The pages which will be mapped are allocated in nvmet_tcp_map_data(),
using the GFP_KERNEL flag. This assures that they cannot come from
HIGHMEM. This imply that a straight page_address() can replace the kmap()
of sg_page(sg) in nvmet_tcp_map_pdu_iovec(). As a side effect, we might
also delete the field "nr_mapped" from struct "nvmet_tcp_cmd" because,
after removing the kmap() calls, there would be no longer any need of it.

In addition, there is no reason to use a kvec for the command receive
data buffers iovec, use a bio_vec instead and let iov_iter handle the
buffer mapping and data copy.

Test with blktests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

[1] "[PATCH] checkpatch: Add kmap and kmap_atomic to the deprecated
list" https://lore.kernel.org/all/[email protected]/

Cc: Chaitanya Kulkarni <[email protected]>
Cc: Keith Busch <[email protected]>
Suggested-by: Ira Weiny <[email protected]>
Signed-off-by: Fabio M. De Francesco <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Suggested-by: Al Viro <[email protected]>
[sagi: added bio_vec plus minor naming changes]
Signed-off-by: Sagi Grimberg <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>

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