LU-5389: optimize ll_fid2path()

LU-5389: optimize ll_fid2path()

The only parameter from userspace that matters is the length of the
buffer. We don't need to allocate then import the whole structure. By
importing only that length, we can save a memory allocation.

Add sparse annotations to that function.

Change-Id: I4baa941f0128f9bcc19fade112915f3c52cd5b8b
Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/11167
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>