JFFS2: Improve speed reading flash files

jffs2_1pass_read_inode() would read the entire data for each node
in the filesystem, regardless of whether it was part of the file
to be loaded or not. By only reading the header data for an inode,
and then reading the data only when it is found to be part of the
file to be loaded, much copying of data is saved.

jffs2_1pass_list_inodes() read each inode for every file in the
directory into a buffer. By using NULL as a buffer pointer, NOR
flash simply returns a pointer, and therefore avoids a memory copy.

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