zram: rename internal slot API

zram: rename internal slot API

We have a somewhat confusing internal API naming.  E.g.  the following
code:

	zram_slot_lock()
	if (zram_allocated())
		zram_set_flag()
	zram_slot_unlock()

may look like it does something on zram device level, but in fact it tests
and sets slot entry flags, not the device ones.

Rename API to explicitly distinguish functions that operate on the slot
level from functions that operate on the zram device level.

While at it, fixup some coding styles.

[senozhatsky@chromium.org: fix up mark_slot_accessed()]
Link: https://lkml.kernel.org/r/20260115031922.3813659-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/775a0b1a0ace5caf1f05965d8bc637c1192820fa.1765775954.git.senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: David Stevens <stevensd@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>