pipe_fs_i.h: fix kernel-doc warning

Fix kernel-doc notation warnings in pipe_fs_i.h:

  Warning(include/linux/pipe_fs_i.h:58): No description found for parameter 'buffers'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>