include/linux/uaccess.h: remove expensive WARN_ON in pagefault_disabled_dec

pagefault_disabled_dec is frequently used inline, and it has a WARN_ON
for underflow that expands to about 6.5k of extra code.  The warning
doesn't seem to be that useful and worth so much code so remove it.

If it was needed could make it depending on some debug kernel option.

Saves ~6.5k in my kernel

     text    data     bss     dec     hex filename
  9039417 5367568 11116544        25523529        1857549 vmlinux-before-pf
  9032805 5367568 11116544        25516917        1855b75 vmlinux-pf

Link: http://lkml.kernel.org/r/20170315021431.13107-8-andi@firstfloor.org


Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>