bluefs: bluefs alloc unit should only be shrink

bluefs: bluefs alloc unit should only be shrink

The alloc unit has already forbidden changed for bluestore, what's more,
it should forbidden increased in bluefs. Otherwise, it can leads to
coredump or bad data. Let's explain it use Bitmap Allocater, it has two
presentations:
a) in BitmapAllocator::init_rm_free(offset, length),
(offset + length) should bigger than offs. But when get_min_alloc_size()
changed bigger, this can not be guaranteed.
b) if init_rm_free() are
successfully in luck, then in rocksdb compact, when release() be called,
it release a small extent but may leads to larger extents be released to
Bitmap. As a result, rocksdb data is corrupted, and the osd can not be
booted again.

Signed-off-by: Mingyuan Liang <liangmingyuan@baidu.com>