Do not allocate struct statfs on kernel stack.
Right now size of the structure is 472 bytes on amd64, which is
already large and stack allocations are indesirable. With the ino64
work, MNAMELEN is increased to 1024, which will make it impossible to have
struct statfs on the stack.
Extracted from: ino64 work by gleb
Discussed with: mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week