LU-13096 llite: fix potential overflow in ll_max_cached_mb_seq_write()

LU-13096 llite: fix potential overflow in ll_max_cached_mb_seq_write()

atomic_long_cmpxchg() return long, however @rc is int, if we have
a larger memory etc 24T, we will get overflow, and @diff will never
change thus we get a hang loop there.

Test-Parameters: trivial
Fixes: adb5aca3d673 ("LU-8066 llite: Move all remaining procfs entries to debugfs")
Change-Id: I20d6feff9797ba10a089587bee0d8691bee460df
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37707
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>