This reverts commit 36811b4430aaea8c8b91bbe7d812a26799865499.
Which is no longer required because there is now SPL code in
place to safely handle the deadlocks the kernel patch was designed
to address. Therefore we can unconditionally use vmalloc() and
drop all the PF_MEMALLOC code.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
bc03e07a7c