Bug fix: Hold spinlock across find|remove /sys/firmware/memmap/X operation.

Bug fix: Hold spinlock across find|remove /sys/firmware/memmap/X operation.

It is unsafe to return an entry pointer and release the map_entries_lock.
So we should not hold the map_entries_lock separately in
firmware_map_find_entry() and firmware_map_remove_entry().  Hold the
map_entries_lock across find and remove /sys/firmware/memmap/X operation.

And also, users of these two functions need to be careful to hold the lock
when using these two functions.

The suggestion is from Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>