vfs: improve comment describing fget_light()

Improve the description of fget_light(), which is currently incorrect
about needing a prior refcnt (judging by the way it is actually used).

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>