f2fs: don't handle error case of f2fs_compress_alloc_page()

f2fs_compress_alloc_page() uses mempool to allocate memory, it never
fail, don't handle error case in its callers.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>