buf_dblwr_create(): Remove a bogus check for the buffer pool size.
Theoretically, there is no problem if the doublewrite buffer is
larger than the buffer pool. It could only cause trouble on crash
recovery, and on recovery the doublewrite buffer is read to a buffer
that is allocated outside of the buffer pool. Moreover, this check
was only performed when the database was initialized for the first
time.

On a normal startup, buf_dblwr_init() would not enforce any
rule on the innodb_buffer_pool_size.

Furthermore, in case of an error, commit the mini-transaction in order
to avoid an assertion failure on shutdown. Yes, this will leave the
doublewrite buffer in a corrupted stage, but the doublewrite buffer
should only be initialized when the data files are being initialized
from the scratch in the first place.