git-p4: honor lfs.storage configuration variable

git-p4: honor lfs.storage configuration variable

"git lfs" allows users to specify the custom storage location with
the configuration variable `lfs.storage`, but when interacting with
GitLFS pointers, "git p4" always uses the hardcoded default that is
the `.git/lfs/` directory, without paying attention to the
configuration.

Use the value configured in `lfs.storage`, if exists, as all the
"git" operations do, for consistency.

Signed-off-by: r.burenkov <panzercheg@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>