provide generic_block_fiemap() only with BLOCK=y

This fixes the following compile error with CONFIG_BLOCK=n caused by
commit 68c9d702

 ("generic block based
fiemap implementation"):

    CC      fs/ioctl.o
  fs/ioctl.c: In function 'generic_block_fiemap':
  fs/ioctl.c:249: error: storage size of 'tmp' isn't known
  fs/ioctl.c:272: error: invalid application of 'sizeof' to incomplete type 'struct buffer_head'
  fs/ioctl.c:280: error: implicit declaration of function 'buffer_mapped'
  fs/ioctl.c:249: warning: unused variable 'tmp'
  make[2]: *** [fs/ioctl.o] Error 1

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>