jffs2: GC deadlock reading a page that is used in jffs2_write_begin()

GC task can deadlock in read_cache_page() because it may attempt
to release a page that is actually allocated by another task in
jffs2_write_begin().
The reason is that in jffs2_write_begin() there is a small window
a cache page is allocated for use but not set Uptodate yet.

This ends up with a deadlock between two tasks:
1) A task (e.g. file copy)
   - jffs2_write_begin() locks a cache page
   - jffs2_write_end() tries to lock "alloc_sem" from
	 jffs2_reserve_space() <-- STUCK
2) GC task (jffs2_gcd_mtd3)
   - jffs2_garbage_collect_pass() locks "alloc_sem"
   - try to lock the same cache page in read_cache_page() <-- STUCK

So to avoid this deadlock, hold "alloc_sem" in jffs2_write_begin()
while reading data in a cache page.

Signed-off-by: Kyeong Yoo <kyeong.yoo@alliedtelesis.co.nz>
Signed-off-by: Richard Weinberger <richard@nod.at>