logfs: fix build warning

fs/logfs/dev_bdev.c: In function '__bdev_writeseg':
include/linux/kernel.h:601:17: warning: comparison of distinct pointer types lacks a cast [enabled by default]
  (void) (&_min1 == &_min2);  \
fs/logfs/dev_bdev.c:84:14: note: in  expansion of macro 'min'
  max_pages = min(nr_pages, BIO_MAX_PAGES);

fs/logfs/dev_bdev.c: In function 'do_erase':
include/linux/kernel.h:601:17: warning: comparison of distinct pointer types lacks a cast [enabled by default]
 (void) (&_min1 == &_min2);  \
fs/logfs/dev_bdev.c:174:14: note: in expansion of macro 'min'
 max_pages = min(nr_pages, BIO_MAX_PAGES);

Lets use min_t and mention the type.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Cc: Joern Engel <joern@logfs.org>
Cc: Prasad Joshi <prasadjoshi.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>