LU-8774 lprocfs: not use MAX_STRING_SIZE in copy_from_user

LU-8774 lprocfs: not use MAX_STRING_SIZE in copy_from_user

This patch removes the usage of MAX_STRING_SIZE from
copy_from_user() and just copies enough bytes to cover
count passed in.

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I1ac2c779b5cd984f88bb85d4ae8d571f7931091f
Reviewed-on: http://review.whamcloud.com/23462
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>