ANDROID: Incremental fs: Use R/W locks to read/write segment blockmap.

Use Read-Write locks for reading/writing segment in blockmap.
This should allow parallel reads when there are
multiple reads within same segment.

A small optimization in pending_reads_read(). Since
incfs_collect_pending_reads() already iterate to
populate buffer, new_max_sn - highest serial number
among all the pending read buffer can be done in the same
loop instead of looping again in pending_reads_read().

Bug: 161566104
Test: kernel selftest - incfs_test and incfs_perf

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Id00376b0e4cb8c0c0bc8264cdddd6f38c4aa85f0