xfs: use NOIO contexts for vm_map_ram

When we map pages in the buffer cache, we can do so in GFP_NOFS
contexts. However, the vmap interfaces do not provide any method of
communicating this information to memory reclaim, and hence we get
lockdep complaining about it regularly and occassionally see hangs
that may be vmap related reclaim deadlocks. We can also see these
same problems from anywhere where we use vmalloc for a large buffer
(e.g. attribute code) inside a transaction context.

A typical lockdep report shows up as a reclaim state warning like so:

[14046.101458] =================================
[14046.102850] [ INFO: inconsistent lock state ]
[14046.102850] 3.14.0-rc4+ #2 Not tainted
[14046.102850] ---------------------------------
[14046.102850] inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-W} usage.
[14046.102850] kswapd0/14 [HC0[0]:SC0[0]:HE1:SE1] takes:
[14046.102850]  (&xfs_dir_ilock_class){++++?+}, at: [<791a04bb>] xfs_ilock+0xff/0x16a
[14046.102850] {RECLAIM_FS-ON-W} state was registere...