setextattr: Increase stdin buffer size to 4096

Extended attribute values can potentially be quite large. One test for ZFS
is supposed to set a 200MB xattr. However, the buffer size for reading
values from stdin with setextattr -i is so small that the test times out
waiting for tiny chunks of data to be buffered and appended to an sbuf.

Increasing the buffer size should help alleviate some of the burden of
reallocating larger sbufs when writing large extended attributes.

Submitted by: Ryan Moeller <ryan@freqlabs.com>
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D23211