btrfs: fix a bug writing a partial page when writing an extent

Fix a bug writing a partial page in end_bio_extent_writepage().
The condition "bvec.bv_len != PAGE_CACHE_SIZE" should be not valid any
more, because now biovec is able to handle arbitrary size of bio.
Without this patch, block IO stalls with the following log:

  BTRFS error (device sdb1): partial page write in btrfs with offset 0 and length 8192
  BTRFS critical (device sdb1): bad ordered accounting left 0 size 4096

dpark: replaced min() with min_t() to avoid compiler warning

Signed-off-by: Ming Lin <mlin@minggr.net>