NFS4: Avoid NULL reference or double free in nfsd4_fslocs_free()

If fsloc_parse() failed at kzalloc(), fs/nfsd/export.c
 411
 412         fsloc->locations = kzalloc(fsloc->locations_count
 413                         * sizeof(struct nfsd4_fs_location), GFP_KERNEL);
 414         if (!fsloc->locations)
 415                 return -ENOMEM;

svc_export_parse() will call nfsd4_fslocs_free() with fsloc->locations = NULL,
so that, "kfree(fsloc->locations[i].path);" will cause a crash.

If fsloc_parse() failed after that, fsloc_parse() will call nfsd4_fslocs_free(),
and svc_export_parse() will call it again, so that, a double free is caused.

This patch checks the fsloc->locations, and set to NULL after it be freed.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>