f2fs: fix sparse warnings

f2fs contained a number of endianness conversion bugs.

Also, one function should have been 'static'.

Found with sparse by running 'make C=2 CF=-D__CHECK_ENDIAN__ fs/f2fs/'

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>