btrfs: create helper btrfs_find_device_by_user_input()

btrfs: create helper btrfs_find_device_by_user_input()

The patch renames btrfs_dev_replace_find_srcdev() to
btrfs_find_device_by_user_input() and moves it to volumes.c, so that
delete device can use it.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Orabug: 26287586

(Cherry picked from commit 24e0474b59538cdb9d2b7318ec7c7ae9f6faf85d)

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>