nfsd: split off the write decode code into a separate function

nfs4_decode_write has code to parse incoming XDR write data in to
a kvec head, and a list of pages.

Put this code in to a separate function, so that it can be used
later by the xattr code, for setxattr. No functional change.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>