Remove writability requirement for single-mbuf, contiguous-range
m_pulldown()

m_pulldown() only needs to determine if a mbuf is writable if it is going to
copy data into the data region of an existing mbuf. It does this to create a
contiguous data region in a single mbuf from multiple mbufs in the chain. If
the requested memory region is already contiguous and nothing needs to
change, the mbuf does not need to be writeable.

Submitted by: Brian Mueller <bmueller@panasas.com>
Reviewed by: bz
MFC after: 1 week
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D9053