Fix stack overflow in vn_rdwr() due to memory reclaim

Fix stack overflow in vn_rdwr() due to memory reclaim

Unless __GFP_IO and __GFP_FS are removed from the file mapping gfp
mask we may enter memory reclaim during IO.  In this case shrink_slab()
entered another file system which is notoriously hungry for stack.
This additional stack usage may cause a stack overflow.  This patch
removes __GFP_IO and __GFP_FS from the mapping gfp mask of each file
during vn_open() to avoid any reclaim in the vn_rdwr() IO path.  The
original mask is then restored at vn_close() time.  Hats off to the
loop driver which does something similiar for the same reason.

  [...]
  shrink_slab+0xdc/0x153
  try_to_free_pages+0x1da/0x2d7
  __alloc_pages+0x1d7/0x2da
  do_generic_mapping_read+0x2c9/0x36f
  file_read_actor+0x0/0x145
  __generic_file_aio_read+0x14f/0x19b
  generic_file_aio_read+0x34/0x39
  do_sync_read+0xc7/0x104
  vfs_read+0xcb/0x171
  :spl:vn_rdwr+0x2b8/0x402
  :zfs:vdev_file_io_start+0xad/0xe1
  [...]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>