LU-17566 mdt: remove duplicate call to mdt_init_ucred_reint()

LU-17566 mdt: remove duplicate call to mdt_init_ucred_reint()

Remove duplicate call to mdt_init_ucred_reint() from
mdt_reint_setxattr().

mdt_init_ucred_reint() is called in mdt_reint_internal() which is
covering all actual reinters. However, SETXATTR was converted to
reinters framework in fd908da and this call was not removed.
So mdt_init_ucred_reint() is called first in mdt_reint_internal() then
again in the specific mdt_reint_setxattr() handler, without anything
special being done on the ucred between them.

Also merge __mdt_init_ucred() and mdt_init_cred() which was
called only once, and with the same prototype.

Lustre-change: https://review.whamcloud.com/54111
Lustre-commit: 65e0802f2ada98f802d01b5672bb9349ad0dde8c

Signed-off-by: Aurelien Degremont <adegremont@nvidia.com>
Change-Id: I90fed1d2709edf7337a27dd9c3cb0f75f7625135
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54368
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>