ksmbd: return requested create allocation size

smb2.create.blob sends an SMB2_CREATE_ALLOCATION_SIZE create context with
a 1MiB allocation size and expects the create response AllocationSize field
to match the requested size.  smb2.create.open additionally compares the
AllocationSize returned in the CREATE response with the AllocationSize
returned by FILE_ALL_INFORMATION on the same handle.

ksmbd applies the allocation with fallocate(), but then fills both the
create response and handle-based information from stat.blocks << 9.  On
filesystems such as ext4 this can include filesystem allocation rounding
and metadata effects, causing a response larger than the SMB2 allocation
size context and a disagreement between the two queries.

Remember the requested allocation size while processing the create context,
store the reported allocation size in struct ksmbd_file, and use it for
both the create response and handle-based allocation size responses. Update
the stored value when FILE_ALLOCATION_INFORMATION changes it, and fall back
to stat.blocks << 9 when no allocation size context was provided.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

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