configure: add check for bswap_64

include/f2fs_fs.h checks the HAVE_BSWAP_64 conidtional, but configure
nevers checks for it.

Add a check that the function is indeed declared, and fix the variable
name to match the naming scheme of autoconf (and not those of WAF like
is used by samba), and adapt the check as suggested in the autoconf
manual.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
2 files changed