seq_file: don't call bitmap_scnprintf_len()

"m->count + len < m->size" is true commonly, so bitmap_scnprintf()
is commonly called. this fix saves a call to bitmap_scnprintf_len().

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Paul Menage <menage@google.com>
Cc: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>