fs: remove stale and duplicate forward declarations

fs: remove stale and duplicate forward declarations

Remove the following unnecessary forward declarations from fs.h, which
improves maintainability.

- struct hd_geometry: became unused in fs.h when
  block_device_operations was moved to blkdev.h in commit 08f858512151
  ("[PATCH] move block_device_operations to blkdev.h"). The forward
  declaration is now added to blkdev.h where it is actually used.

- struct iovec: became unused when aio_read/aio_write were removed in
  commit 8436318205b9 ("->aio_read and ->aio_write removed")

- struct iov_iter: duplicate forward declaration. This removes the
  redundant second declaration, added in commit 293bc9822fa9
  ("new methods: ->read_iter() and ->write_iter()")

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512301303.s7YWTZHA-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202512302139.Wl0soAlz-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202512302105.pmzYfmcV-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202512302125.FNgHwu5z-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202512302108.nIV8r5ES-lkp@intel.com/
Signed-off-by: Yuto Ohnuki <ytohnuki@amazon.com>
Link: https://patch.msgid.link/20260226201857.27310-2-ytohnuki@amazon.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>