LU-17000 mdt: symbolic link integer overflow

LU-17000 mdt: symbolic link integer overflow

CoverityID: 442376 ("Integer overflow")
Use ssize_t to avoid narrowing

CoverityID: 442114 ("Integer handling issues")
mdt_getattr_internal() a theoretical underflow when subtracting
from an unsigned int that could be 0

Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I560c9cbcbfed907725b101ca90661fe8d0d6cf45
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56151
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>