rbd: use MIN instead of min()

rbd: use MIN instead of min()

Not even sure where min() was coming from, but it seems to be missing on
i386 lucid.:

g++ -DHAVE_CONFIG_H -I.     -Wall -D__CEPH__ -D_FILE_OFFSET_BITS=64 -D_REENTRANT -D_THREAD_SAFE -rdynamic -g -O2 -MT rbd.o -MD -MP -MF .deps/rbd.Tpo -c -o rbd.o rbd.cc
rbd.cc: In function 'int do_import(void*, const char*, int, const char*)':
rbd.cc:837: error: no matching function for call to 'min(uint64_t&, off_t)'
make[3]: *** [rbd.o] Error 1

Reported-by: John Leach <john@johnleach.co.uk>
Signed-off-by: Sage Weil <sage@newdream.net>