mm/gup.c: fix coding style issues.

- Fixed style error: 8 spaces -> 1 tab.
- Fixed style warning: Corrected misleading indentation.

Link: http://lkml.kernel.org/r/20180302210254.31888-1-marioleinweber@web.deSigned-off-by: NMario Leinweber <marioleinweber@web.de>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>