JFFS2: Return early when file read not necessary

If a destination is not provided, jffs2_1pass_read_inode() only
returns the length of the file. In this case, avoid reading all
the data nodes, and return as soon as the length of the file is
known.

Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>