lib/bitmap.c: fix remaining space computation in bitmap_print_to_pagebuf

lib/bitmap.c: fix remaining space computation in bitmap_print_to_pagebuf

For various alignments of buf, the current expression computes

4096 ok
4095 ok
8190
8189
...
4097

i.e., if the caller has already written two bytes into the page buffer,
len is 8190 rather than 4094, because PTR_ALIGN aligns up to the next
boundary.  So if the printed version of the bitmap is huge, scnprintf()
ends up writing beyond the page boundary.

I don't think any current callers actually write anything before
bitmap_print_to_pagebuf, but the API seems to be designed to allow it.

[akpm@linux-foundation.org: use offset_in_page(), per Andy]
[akpm@linux-foundation.org: include mm.h for offset_in_page()]
Link: http://lkml.kernel.org/r/20180818131623.8755-7-linux@rasmusvillemoes.dk
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Yury Norov <ynorov@caviumnetworks.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>